Changes needed to change external ports

I recently changed my ISP for another one, after that my nextcloud server behind it stopped working because the new router has the ports 80 and 443 in use for its own and it doesn’t let me assign those ports to the server.
I am using the vm image of nextcloud 15, fully updated as of right now.
I have a ddns with a domain name and a working ssl certificate by certbot.

I want to know which changes should I make in the config files for apache and nextcloud to make it work, because I have redirected two external ports, say 81 and 4443 to the server’s internal ports 80 and 443, but whenever I try to access my server it redirects to the router’s admin page.

I think I am missing something, perhaps some redirect to the ddns name using the external ports, but I am not sure where should I put that configuration.

Any chance to change the router?
Using port 80 and 443 (more important) for its own is just bad.