Is this Three's 5g home broadband?
If so, try going into your router's APN settings and change the mode to Manual, the profile to "Custom" and the PDP type from the default IPv4v6 to IPv4 (this is with the ZTE router, I presume the Zyxel will be similar). Also make sure the APN is "3internet" and not "three.co.uk" otherwise you'll probably be on CGNAT which will cause port forwarding issues.