No internal access to nextcloud

I installed nextcloud following the fantastic TechnoDadLife videos (thanks!) and it works perfectly. After that I followed his other video to, thanks to letsencrypt, be able to access safely from the outside.
The problem I have now is that I can access from the outside, but not from the internal network, automatically redirects me to the subdomain of duckdns.org and not to the internal ip.
Any clue where the problem may be or how to solve it?
Thanks.

Thanks for the tip gderf !

I will find out if my router (Huawei 566a) allows NAT reflection.
It would be easier since I would not have to edit the host file on each machine.

Or could it be solved by configuring NextCloud?