What could be the reason for the constant breaks Mikrotik + MF652 + MTS?

Question

Mikrotik

Welcome all.
There is a 3G modem MF652 from MTS. There is a local network of three computers (Win7) + several smartphones. There is a Mikrotik RB951G-2HnD router.
If you plug the modem into one of the computers, share the connection using Windows tools, configure network routing through this computer, then everything works fine. The connection is stable, lasts for days.
If you plug the modem into the router, then as soon as you allow it to forward (masquerade is enabled) traffic from the network, then disconnects immediately begin on the initiative of MTS (Normal termination by NCP). If you leave one router, then the connection also keeps stable for days and does not fall: as soon as transit traffic appears, it will disconnect.
What could be the problem?

Answer 1

In general, the problem was solved by adding a rule that drops all "invalid" packets. Why these packets led to a disconnection is not clear, if possible I will conduct a deeper study.
Thank you all for your help.

Answer 2

1) If you turn on the proxy on Mikrotik and walk through it, do breaks occur?
2) Does increasing the TTL for transit packets in the /ip firewall mangle help?

Answer 3

I saw this with openvpn. Check two things
1) That the ppp settings indicate exactly the usb port that the Mikrotik is connected to
2) Try turning on the modem and running ping. Will the connection break? Are the pings normal?
3) Try playing with channels (data channel, info channel) in ppp settings.