just a hunch, but do you get some sort of IPv6 messages when you ping that D DNS name “rglongpre.ddns.net” from the host command line?
… bacause that setup is much simpler than Nextcloud + Docker AIO + domain validation.
maybe the in-laws have a NATed connection? see What CG-NAT is, how to detect it, why it is bad, what can you do about it