IPSec connection from the blue network to an external server on the wan blocked by the firewall

Some time during the last quarter of 2019 I realized I could not connect anymore from an iOS phone to an external IPSec server (IKEv1), which it worked perfectly before. The connection works when IPFire is not involved. I recently started investigating the issue and I noticed that upon trying to establish a connection, the kernel emits a log with DROP_INPUT of the inbound SYN packet coming from the IPSec server.

I assume some rule of the firewall has changed sometime in the last quarter of 2019 and now it blocks the connection. Unfortunately both source and destination ports change every time and the server is a Fritzbox machine with a dynamic DNS kind of setting, so the IP changes as well. This prevents me from having the possibility to open the firewall with the WUI.

Any suggestion on how to solve this issue will be much appreciated.