[OpenAFS] Networking AFS related problem

[renyhp]

Hello,

I'm facing an issue where, if I am connected to a specific WiFi
(mobile) hotspot, any command like stat, ls, cd etc, on an AFS mount,
hangs indefinitely. My /var/log/kern.log is filled with messages like

afs: Lost contact with file server X.X.X.X in cell example.com
(code -1) (all multi-homed ip addresses down for the server)

Also, fs checkservers gives error for the servers of my cell.

I am on Ubuntu 20.04.3, with kernel 5.13.0-27-generic, openafs-client
installed via apt, version 1.8.4~pre1-1ubuntu2.3.

Since AFS is working perfectly as soon as I change my WiFi connection
to other connections, and the cell is perfectly reachable via a remote
machine, I think the problem is with my ISP, but I don't have enough
knowledge about AFS or networking to pinpoint the problem precisely.
(Of course, "normal" web browsing is perfectly okay even with this
problematic connection.)

I would be very grateful if anyone can help me gather some more
debugging info about this problem so I can give it to my ISP for them
to fix it.

Thank you in advance!