VPN connection issue

Hello,

I have two sim contracts:

A) sim contract unlimited 5G (no public IP), which I am using as a hotspot for my laptop when I’m abroad

B) sim contract unlimited 5G (with public IP, activation paid, and APN set on netpublic), that I am using to host a remote VPN service.

On B, which has a dynamic public IP address, I set up a DDNS service that works perfectly. Periodically, my DNS name gets updated as expected with the public IP that the router receives via DHCP.

If I am using A, I can't connect to my service and cannot even ping the router.

Device A has no public IP address, but it gets an IP address from the cell like 10.178.7.176, and an external one like 176.2.66.7.

The same service I host on connection B can work just fine from other connections that are not from O2. For example, I tried other mobile connections of other ISPs, and I could both ping router B and connect to my VPN service.

I’ve checked the connection of device A, and it works just fine. I can ping Cloudflare, Google, and speed over 5G is good, and I have no issues with normal internet connectivity (APN is set as standard over NAT, public).

To debug the issue, I ran a traceroute from A to B, which stopped at some point in O2’s network.

$ traceroute <my dns name>

1 10.0.80.195 23.9ms

2 10.81.102.9 28.3ms

3 ...

And I monitored the packets on B coming from the WAN interface. When pinging or sending other traffic from A to B, no packets were received, whereas the same tests from a different connection outside O2’s network were successful. It seems like packets are discarded in the O2 network.

It seems to me that packets from A under NAT, while traveling through O2’s network, and the destination is under O2’s network with a public IP (device B), are dropped before reaching the destination.

Has anyone had any ideas on what is going on and how to solve this?

Thank you very much in advance.