I'm a colleague of gary.erx, and as he's out of office I'll try to summarize our investigations so far on this case.
We did multiple tests, with three different operators, in two different locations (we have two offices), and with firmware 2.6.1, 7.3.3 and 7.4.3. We tried to reproduce systematically the issue, using two types of connections. We have a device runing Ubuntu attached to the Lan of the router, running Wireguard client, and also using rclone from this device to outside (rclone target doesn't matter, problem is the same with S3, Google Drive, ...).
As a conclusion, we observed that the Location is more important than any other parameters, as at one office's location everything is OK (for every operator and protocol), and at the other one, depending on the firmware version, either ssh through wireguard OR rclone is not working.
We struggle to understand how this is supposed to work, as obviously in addition of the operator the antenna seems to have an active role.
We would appreciate any help you may provide, thanks !
Update: After having a call to an engineer working for an GSM operator, I realized that the number of 4G frequencies available on each antenna is different, and therefore the bandwidth. The router should then negociate the right MTU using MSS, and this seems to be not well working on recent firmwares (our guess).
Is there any other "internal" commands we might use to diagnosticate this deeper ?