Help with setting up my network

I’m going to skip backstory because I’m afraid it will confuse things, if ya’ll need it, I’m happy to provide it.

I live in an apartment and have been happy to hardwire basically everything into the modem/router that is attached to the wall (yes, it’s screwed in there and I’m not allowed to touch it per my lease). However, I just got a new WFH job that requires me to remove all computers and electronics from whatever room I’m working in. So now I’m setting up a network for the first time.

Reception outside this room is exceptionally poor so I have the bright idea to basically DIY a mesh network. I own a Netgear R9000 flashed with dd-wrt. I assume I need to put it in bridge mode to avoid double NAT. I have no idea how to set this up. I can log in the the GUI and can view the settings and even know where the settings are but I don’t know what each setting does precisely. My life would be a lot simpler if I could leave it in AP mode and adjust settings as I RTFM. The documentation on dd-wrt wiki is rather poor unfortunately so that’s why I’m here.

Ultimately and ideally, I would like to have my own network that I can control without introducing issues. I would prefer using the network that I share with other residents as it is bundled with my rent but if I need to pay xfinity or whomever to get my own connection to set this up, then so be it.

Thoughts?

Update: So I’ve discovered that if I connect the gateway to my Netgear router via its LAN port instead of WAN port this bypasses the double NAT problem? Correct? If so, can I use the router in AP mode?