I have a Dell branded Intel X710-T4L card that advertises only 1G and 10G modes even though it should also work with 5G and 2.5G. I guess the Dell firmware might be the cause of this problem.
Now I would like to flash the Intel firmware.
Could you provide more detailed instructions how to do that? I don’t want to brick the card for obvious reasons. 
root@proxmox:~# ethtool enp5s0f1
Settings for enp5s0f1:
Supported ports: [ ]
Supported link modes: 1000baseT/Full
10000baseT/Full
Supported pause frame use: Symmetric Receive-only
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 1000baseT/Full
10000baseT/Full
Advertised pause frame use: No
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Unknown! (255)
Auto-negotiation: off
Port: Other
PHYAD: 0
Transceiver: internal
Supports Wake-on: d
Wake-on: d
Current message level: 0x00000007 (7)
drv probe link
Link detected: no
root@proxmox:~# ethtool -i enp5s0f1
driver: i40e
version: 6.5.13-1-pve
firmware-version: 9.40 0x8000e9b2 22.5.7
expansion-rom-version:
bus-info: 0000:05:00.1
supports-statistics: yes
supports-test: yes
supports-eeprom-access: yes
supports-register-dump: yes
supports-priv-flags: yes
root@proxmox:~# lspci -d 8086: -vvv
05:00.0 Ethernet controller: Intel Corporation Ethernet Controller X710 for 10GBASE-T (rev 02)
Subsystem: Intel Corporation Ethernet 10G 4P X710-T4L-t Adapter
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
Latency: 0, Cache Line Size: 64 bytes
Interrupt: pin A routed to IRQ 93
NUMA node: 0
IOMMU group: 33
Region 0: Memory at 3001e000000 (64-bit, prefetchable) [size=16M]
Region 3: Memory at 3001f818000 (64-bit, prefetchable) [size=32K]
Expansion ROM at f2680000 [disabled] [size=512K]
Capabilities: [40] Power Management version 3
Flags: PMEClk- DSI+ D1- D2- AuxCurrent=0mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=1 PME-
Capabilities: [50] MSI: Enable- Count=1/1 Maskable+ 64bit+
Address: 0000000000000000 Data: 0000
Masking: 00000000 Pending: 00000000
Capabilities: [70] MSI-X: Enable+ Count=129 Masked-
Vector table: BAR=3 offset=00000000
PBA: BAR=3 offset=00001000
Capabilities: [a0] Express (v2) Endpoint, MSI 00
DevCap: MaxPayload 2048 bytes, PhantFunc 0, Latency L0s <512ns, L1 <64us
ExtTag+ AttnBtn- AttnInd- PwrInd- RBE+ FLReset+ SlotPowerLimit 0W
DevCtl: CorrErr+ NonFatalErr+ FatalErr+ UnsupReq+
RlxdOrd+ ExtTag+ PhantFunc- AuxPwr- NoSnoop- FLReset-
MaxPayload 512 bytes, MaxReadReq 512 bytes
DevSta: CorrErr+ NonFatalErr- FatalErr- UnsupReq+ AuxPwr- TransPend-
LnkCap: Port #0, Speed 8GT/s, Width x8, ASPM L1, Exit Latency L1 <16us
ClockPM- Surprise- LLActRep- BwNot- ASPMOptComp+
LnkCtl: ASPM Disabled; RCB 64 bytes, Disabled- CommClk+
ExtSynch- ClockPM- AutWidDis- BWInt- AutBWInt-
LnkSta: Speed 8GT/s, Width x8
TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-
DevCap2: Completion Timeout: Range ABCD, TimeoutDis+ NROPrPrP- LTR-
10BitTagComp- 10BitTagReq- OBFF Not Supported, ExtFmt- EETLPPrefix-
EmergencyPowerReduction Not Supported, EmergencyPowerReductionInit-
FRS- TPHComp- ExtTPHComp-
AtomicOpsCap: 32bit- 64bit- 128bitCAS-
DevCtl2: Completion Timeout: 260ms to 900ms, TimeoutDis- LTR- 10BitTagReq- OBFF Disabled,
AtomicOpsCtl: ReqEn-
LnkCap2: Supported Link Speeds: 2.5-8GT/s, Crosslink- Retimer- 2Retimers- DRS-
LnkCtl2: Target Link Speed: 8GT/s, EnterCompliance- SpeedDis-
Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
Compliance Preset/De-emphasis: -6dB de-emphasis, 0dB preshoot
LnkSta2: Current De-emphasis Level: -6dB, EqualizationComplete+ EqualizationPhase1+
EqualizationPhase2+ EqualizationPhase3+ LinkEqualizationRequest-
Retimer- 2Retimers- CrosslinkRes: unsupported
Capabilities: [e0] Vital Product Data
Product Name: X710 10GbE Controller
Read-only fields:
[V0] Vendor specific: FFV22.5.7
[PN] Part number: ND4PT
[MN] Manufacture ID: 1028
[V1] Vendor specific: DSV1028VPDR.VER2.1
[V3] Vendor specific: DTINIC
[V4] Vendor specific: DCM1001FFFFFF2101FFFFFF3201FFFFFF4301FFFFFF
[V5] Vendor specific: NPY4
[V6] Vendor specific: PMT1
[V7] Vendor specific: NMVIntel Corp
[V8] Vendor specific: L1D0
[RV] Reserved: checksum good, 1 byte(s) reserved
Read/write fields:
[Y1] System specific: CCF1
End
Capabilities: [100 v2] Advanced Error Reporting
UESta: DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq+ ACSViol-
UEMsk: DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt+ UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq+ ACSViol-
UESvrt: DLP+ SDES+ TLP- FCP+ CmpltTO+ CmpltAbrt- UnxCmplt+ RxOF+ MalfTLP+ ECRC+ UnsupReq- ACSViol-
CESta: RxErr- BadTLP- BadDLLP- Rollover- Timeout- AdvNonFatalErr+
CEMsk: RxErr- BadTLP- BadDLLP- Rollover- Timeout- AdvNonFatalErr-
AERCap: First Error Pointer: 00, ECRCGenCap+ ECRCGenEn- ECRCChkCap+ ECRCChkEn-
MultHdrRecCap- MultHdrRecEn- TLPPfxPres- HdrLogCap-
HeaderLog: 00000000 00000000 00000000 00000000
Capabilities: [140 v1] Device Serial Number e0-d3-f1-ff-ff-91-96-b4
Capabilities: [150 v1] Alternative Routing-ID Interpretation (ARI)
ARICap: MFVC- ACS-, Next Function: 1
ARICtl: MFVC- ACS-, Function Group: 0
Capabilities: [160 v1] Single Root I/O Virtualization (SR-IOV)
IOVCap: Migration- 10BitTagReq- Interrupt Message Number: 000
IOVCtl: Enable- Migration- Interrupt- MSE- ARIHierarchy+ 10BitTagReq-
IOVSta: Migration-
Initial VFs: 32, Total VFs: 32, Number of VFs: 0, Function Dependency Link: 00
VF offset: 16, stride: 1, Device ID: 154c
Supported Page Size: 00000553, System Page Size: 00000001
Region 0: Memory at 000003001f600000 (64-bit, prefetchable)
Region 3: Memory at 000003001f9a0000 (64-bit, prefetchable)
VF Migration: offset: 00000000, BIR: 0
Capabilities: [1a0 v1] Transaction Processing Hints
Device specific mode supported
No steering table available
Capabilities: [1b0 v1] Access Control Services
ACSCap: SrcValid- TransBlk- ReqRedir- CmpltRedir- UpstreamFwd- EgressCtrl- DirectTrans-
ACSCtl: SrcValid- TransBlk- ReqRedir- CmpltRedir- UpstreamFwd- EgressCtrl- DirectTrans-
Capabilities: [1d0 v1] Secondary PCI Express
LnkCtl3: LnkEquIntrruptEn- PerformEqu-
LaneErrStat: 0
Kernel driver in use: i40e
Kernel modules: i40e