rb750gl slow speeds

I use the rb750gl for most home installs. I use the same config on all of them. For 300mbps clients I normally get between 200-280mpbs out of the router. For some reason, only on 1 or 2 routers, I can only get up to 40mpbs, even though there service is 300mpbs out of the modem. Same router, same os, same config. Why would this happen?

Thanks for the help.

Maybe cable or negotiation problem…

I still can’t figure this out. It only happens with certain modems. I can get 300mpbs direct from the modem, but when connected to the router, I only get 100mbps. I checked the wan negotiation, and it only has 100mpbs connection. How can this be? I tried to manually change it to 1000mpps, but when I do that it loses the connection. Is there a way to force everything to be 1000mpbs, or do you have to accept the incoming rate the modem is giving?