VPN only works over cellular internet, but not over wired, what's the problem?

Question

VPN

Good day, has anyone encountered such a problem?:
I set up VPN (L2TP + IPSec) on the Mikrotik RB4011 router. I suffered for a very long time because when I connected from home (provider Rostelecom), the tunnel rose and held on as long as I wanted, until some kind of activity began. As soon as the activity (work in the browser) began, access to the Internet stopped somewhere after 3-5s, after which the tunnel fell. From other providers (cable connection) the same thing, or did not rise at all. But today I tried to connect from mobile Internet (MTS) and the tunnel got up and works stably, friends from Ukraine also tried to connect and it doesn’t work via cable Internet, but it works via mobile.

Answer 1

As soon as activity began (work in the browser), access to the Internet stopped somewhere after 3-5s, after which the tunnel fell . From other providers (cable connection) the same thing, or did not rise at all. But today I tried to connect from the mobile Internet (MTS) and the tunnel got up and works stably

Send diarrhea beams to your wired ISPs, that's them.
If we are talking about Russia, this is a direct consequence of attempts to take control of the Internet, they break standards with crooked hands, block protocols and subnets, interfere with the contents of packets with their left heel. This is our reality, which at best can be changed only through a revolution, which most likely will not help either, but I'm afraid there are no other ways to cure the patient (they are successfully blocked and taken under control).
Try to quarrel with wired providers, first just getting to their admin with logs and a detailed description of what they did, what they expected and what they got. Perhaps they will fix a bug in a homemade DPI and everything will work for you.
Moreover, MTS most likely works only because each provider invents its own special way, and it is buggy for everyone in different ways. And yes, nothing guarantees that VPN will not stop working on MTS tomorrow either.