NFS connection between VPS machines - work out what blocks it/causes timeout

So, I have three VPS servers, which I access with IP addresses. Let's call them A.B.C.1 , A.B.C.2 , and A.B.D.3 . The "D" reflects the fact that it;s a different number - even though it's the same provider and data centre. Information about setting up NFS proved rather hard to find; also authentication seems to require Kerberos setup so I'd rather just restrict access by IP for now (and maybe add Kerberos once it works without Kerberos). I am trying to set up NFS access to a shared directory on A.B.C.1 . A.B.C.1 and A.B.C.2 are running Debian 11 ; A.B.D.3 is running Debian 12. I have put the following line into /etc/exports on A.B.C.1 :

/home/user/shared-dir A.B.C.2(rw,sync,no_subtree_check,all_squash,anonuid=1001,anongid=1001)  A.B.D.3(rw,sync,no_subtree_check,all_squash,anonuid=1001,anongid=1001)

I then made sure the file is read:

# exportfs -a
# systemctl restart nfs-kernel-server

On both client machines, I creates the /home/user/shared-dir-mount directory and then ran:

# mount -t nfs A.B.C.1:/home/user/shared-dir /home/user/shared-dir-mount

This command successfully mounts the share on A.B.C.2 . However, on A.B.D.3 the connection times out. On A.B.D.3, while waiting for the connection to time out, I can see this:

# ss -nt | grep :2049
SYN-SENT 0      1       5.34.179.122:747  5.134.178.122:2049

However on A.B.C.1, the NFS server, when I do ss -tap | grep 2049 , I see nothing. So nothing is actually listening on the port? But then why is the connection working from A.B.C.2 ? No firewall is installed on any of the servers. I'd very much appreciate advice about troubleshooting this one. One guess I have is that it might be a firewall the provider has between subnets or physical boxes (A.B.C.1 and A.B.C.2 might well be on the same physical box); in this case how do I even start trying to bypass this? Another suspicion is some change of defaults between Debian 11 and 12. But all of these are really shots in the dark, and I'm hoping for some expert light here :)