I had this issue with Comcast's Xfinity and my solution was to clone the Mac address of a desktop on the Router.
I tried several desktops/laptops and several routers. All the desktops/laptops got just about full speed. All the routers where pinned at 93.78 or 31.23 for several tests. Cloning the Mac address on the routers that could let the router and devices connected get the full plan speed.
I have a small apache case from harbor freight with a LePotato, mini router and a battery in it. I intended it to mainly be a road trip jellyfin/emby server (still deciding on which to fully implement) but I also have kiwix, Audiobookshelf and a simple nginx webpage to make getting to the services easier.