Have you set up SSL/TLS on Nextcloud? If not, don't even try making it accessible outside your home network. YOU WILL GET OWNED since everything including login username/passwords will be sent in the clear for everyone to see.
As for changing the default port to something else, I'd say don't bother. Use port forwarding on your router instead. Set your WAN incoming port to something random like 46232/TCP to LAN 443/TCP directed at the IP address of your Nextcloud client. That means your parents would type into their browser something like: https://traderjay-home-ip-address:46232
to access Nextcloud from outside your home. You'd still use the same address as you currently do while inside your home network.