Cisco vpn over pptp works crookedly. Where to dig?

Question

VPN

The provider's network consists of LAN and pptp.
My router is connected to the LAN, on which the pptp client is up.
A home computer is connected to the router on the internal interface.
From my home computer, I connect to the office via AnyConnect and work on the office computer via rdp.
Until recently, everything worked. Now the rdp connection is no longer established (although the port is visible, the "telnet ip 3389" test passes).
If you do not raise the pptp client on the router, but run it on your home computer, no problem. We must assume that the problem is in the router, but it gives the impression of a serviceable one, and the same is true with other firmware.
How to bring the provider to clean water? What could they be up to?

Answer 1

MTU needed to be increased. When AnyConnect does not know about pptp, it thinks that I have 1500 and sets 1370 for my network. Why everything worked before is a mystery.
I set 1440 in the router settings and everything worked

Answer 2

There may be problems due to additional encapsulation. Try to reduce the MTU in the vpn settings.