This sounds like it could definitely be an issue with the operator throttling. If that is the case, I'd recommend setting a fixed TTL on the outgoing packets. Configuration example on how this can be done can be found on this thread.
However, there are other ways that the operator could determine if you're using a router, which might not be as easy to work around.
Another thing I'd recommend, is updating to v7.4.1 of RutOS. In the Network -> Firewall -> General make sure the software offloading is enabled.
One last thing that can be tried, is testing a speedtest with the phone hotspot. If you connect the PC to your phone via the hotspot feature, run an extended speed test, and the speeds also drop, this is another good indication.
Operators like to avoid an extended load to their network, so this may not even be a router-related throttling, but rather speed throttling in general after a higher amount of bandwith was used on the device.