When the issue occurs, could you try disconnecting all of the connected devices and check if the speed remains capped with only your PC connected?
If the speed is still limited, would it be possible to connect the TRB directly to your PC, reboot the TRB, and check if the speed still drops after a few hours?
By the looks of it, you have the same issue as I had. This is a known firmware bug. The 12A10 modem update from Quectel is supposed to fix it.
Please find my contact info in the topic linked above if you want it.
Please be advised that this is not an official update. Only use it if you are willing to take the risk. For me, it seems to work, but as Tad noted, it does not work for everyone.
I will let you know when Teltonika support provides me with the firmware with the entire bug correction, but I think it will take a lot of time since he was not aware that the bug was not fixed 100% so the time for Quectel to provide Teltonika one new firmware risk of duration certain period of time