Collabora + Nextcloud = connecting from WAN problem

Hi Guys!

System

  • nextcloud as Truenas plugin (IP adres the same as TrueNAS server → 192.168.1.114 port xyz
  • collabora on VM Ubuntu machine - own IP number like 192.168.1.115
  • on router port forwarding to nextcloud

To install it I used https://www.linuxbabe.com/ubuntu/integrate-collabora-onlinenextcloud-without-docker manual.

On LAN everything works ok!. When I log in to Nextcloud, I can edit documents wit collabora.
But, when I tried to use nextcloud from my public IP, nextcloud say that collabora serwer is inaccessible when I tried edit doc files.
Funny is that, In settings Collabora Online say that collabora server is reachable (192.168.1.115:9980)

What shoul I check first?

The linked guide tells under “Troubleshooting”:

Then edit the /etc/hosts file and add a static DNS record like below. 
Repalce 12.34.56.78 with the public IP address of the server.

12.34.56.78          collaboara.example.com

collabora.example.com should be pointed to the public IP address
 of your server. Don’t point it to 127.0.0.1.

Problem will be, that the public IP address changes from time to time on a dialup-connection