Wireguard problem, worked before but stopped working recently

I am a fairly new user of HA. I have it running on a RPi4 since June. I also set up the wireguard add-on back then without problems. I use DuckDNS (which is updated with the DuckDNS add-on in HA), and obviously a port forward on my router.

This all worked perfectly. I was able to VPN from my Android-phone to Wireguard, and use HA (but also regular internet) through that tunnel. But since some time, this setup doesn’t work anymore.

I can connect to the VPN on my phone. It will state is connected, but I will get no response. When I look at the connection-details on my phone, I see the connection-time and sent packets increase, but packets received remains at zero. In the Wireguard-log in HA, all I see are entries like this:

[17:49:49] INFO: Requesting current status from WireGuard...
interface: wg0
  public key: (removed)
  private key: (hidden)
  listening port: 51820

peer: (removed)
  allowed ips: (removed)
  persistent keepalive: every 25 seconds

I do install updates regularly, so I am on the latest HA and Wireguard-addon version.

Does anybody have any idea how I can start investigating, or does someone have experience with this kind of issue?

Problem solved. For the Google-users ending up here, it turns out my ISP had turned on CGNAT without informing its customers…