I would say to first try the speed on ethernet. If that's slow, then it's the service or the modem and not the router. I think even the worst router you can find would support at least 250 Mbps on Ethernet.

To see if it's the router's fault, you could try some high bandwidth local network transfer, with sftp or something. If that's slow, if you have the money you can just buy one of those fancy gaming routers or some other highly reviewed one.

If there's a few walls or floors in between you and the router that could be the problem and a fancier higher power router will help with that. Another thing that could help is installing another access point near where you're device is, although that's obviously a lot of effort.

If even ethernet is slow and they refuse to help you then if you're in the US or Canada you can try submitting a complaint on the Better Business Bureau website. This actually helped us once or twice when dealing with some cellular problems. You wouldn't think it would do anything but I guess sometimes it gets them to pay at least a little bit of attention to the problem.

I have heard about how bad and monopolistic rural Internet can be, good luck