Have problem if I change from one ISP to the other. Why?

Hello guys, I have a problem with my internet connection which drives me crazy. I would appreciate any help.

I have fiber connection from ISP (lets call it ISP1). ISP1 supplied me a Huawei EchoLife HG8010H V2 GPON and a ZTE router. However, as ISP1 doesn't give me PPOE credentials and there is no bridging in their router, I had to put the router in DMZ mode and directed all traffic to my pfSense (VM). This works. But, I am not quite happy with double NAT.

So, I got a second fiber line from another ISP, ISP2, with intention of dropping ISP1 altogether. ISP2 supplied me NOKIA G-240G-C GPON and a ZTE router. As ISP2 gives PPPOE credentials, I put their router in bridge, entered PPPOE credentials in pfSense. This also works.

Both connections (ISPs) are available at the moment.

Now, on to the problem. I am running Nextcloud + Collabora with Letsencrypt certs (all as dockers on Unraid). While I am connected to ISP1, all works fine. While I am connected to ISP2, Collabora doesn't open in Nextcloud. The errors come up are related to SSL certificates. However, both Nextcloud and Collabora works independently.

I am getting Letsencrypt certs as wildcard using Cloudflare plugin. Of course, when I change from ISP1 to ISP2, or vice versa, I change the static IP on Cloudflare DNS settings, accordingly.

More info:

1. I don't know how to enter Nokia GPON as I don't know the login/pass. So, I am not sure what is the configuration of GPON.
2. I turned off all security related settings on ZTE router.

Anybody has any idea why this happens.