I bought a pair of USB 3.0 to GBit Ethernet adapters hoping i could connect 2 Linux Laptops direct because they are not always on the same network due to dodgy internet connections here. One is usually connected to ADSL and the other is connected to a LTE router but when i want transfer files then i have to disconnect the one that is connected to the LTE router and connect it to the ADSL router. So i was hoping to use two USB to ethernet adapters to permanently connect both laptops for file transfer but only the older Laptop with Ubuntu 17.10 laptop seem to recognize these adaptors but not my Ubuntu 18.04 Laptop. I use the command lsusb before plugging them in and again afterwards but it dont show up (BTW the link and act led’s light up when plugging an ethernet cable so they seem to work, just not recognised by Ubuntu 18.04) BTW it comes up as a Realtek on the Ubuntu 17.10 laptop.
you try running a live cd on the 18.04 laptop with something closer to the 17.10 version or 18.10 to see if its just something weird on the 18.04 build or the hardware ?
Have you tried swapping them between the two machines, and swapping the non-working one to a different USB port, even a USB 2 port?
Try dmesg -Tw while you plug them in.
Look in /etc/modprobe.d/* and see if they might be blacklisted?