Hello @ConnorY,
welcome to the Nextcloud community 
Unfortunately you have not completed the required support template.
Please add config files, logs and error messages from you tries. Without additional information the community members cannot help you.
In general you don’t want to have different URLs - reasons have been discussed many times already local_network and best practise is splitbraindns as you already tried. please review 101: Split-Brain DNS (split-horizon) and add details. I could imagine you are using Cloudflare as reverse proxy - which obviously makes use of split-brain DNS impossible - you must configure your local Nginx with TLS certificate and use this IP as an entry point.
Regards,
wwe