Try searching for trusted domains! Number one referenced topic on the forum.
https://help.nextcloud.com/search?q=howto%20trusted%20domain
KarlF12
3
Also have a look at the documentation, where it explains this setting as well as how to change it both using occ and by hand.
https://docs.nextcloud.com/server/stable/admin_manual/
Thanks but I need a step by step simple instructions, maybe using putty to get this done. The online info looks like Klingon to me.