Debugging PPPoSSH

Hallo all

I'm trying to use PPPoSSH to connect a mobile OpenWRT device (let's call it A) to another OpenWRT device in my home network (let's call it B).

I know, SSH is not a good tunneling protocol, and I'll be using Wireguard whenever I can, but there are some countries in the world that block Wireguard but allow SSH.

I've tried to follow the instructions here to the best of my knowledge: https://openwrt.org/docs/guide-user/services/vpn/pppossh/start

I can login via SSH to A, and from there on to B, so there seems to be nothing in the way of the SSH connection itself. That works with both password as well as the key configured in the PPPoSSH config on device A.
Whenever I do that, I can see a successful login in the logs of B.

I can even run a PPPD command on device B quite similar to what I can reconstructed from here: https://github.com/openwrt/packages/blob/2f38de7fface18b4b417938a79108e6834027df6/net/pppossh/files/pppossh.sh#L65
The result is funny characters, so something seem to be doing something.

But when I click "Restart" on the PPPoSSH interface on device A, it doesn't work:
The device will show " Error: Network device is not present".
There is only this in the log of A:

Sun Jul  6 15:40:14 2025 daemon.notice netifd: Interface 'vpn' is setting up now
Sun Jul  6 15:40:14 2025 daemon.notice netifd: Interface 'vpn' is now down
Sun Jul  6 15:40:14 2025 user.notice firewall: Reloading firewall due to ifdown of vpn ()

And nothing at all in the logs of B, not even a failed login attempt. As if device A is not even trying.

So, I guess, something is not right on device A, but I don't know what.

Maybe someone can give me some hints where to look or what to try to get more information about what goes wrong?