I have the port forwarding enabled as well as a reverse proxy (Nginx Proxy Manager) running on my LAN. I can access the subdomains without issues while using cellular data.
I do not want to buy a new modem just for this feature and I don't want to have to use my DNS Server to trick my LAN clients into using Local DNS records..
Telus has that disabled for all residential connections regardless of modem used. I've asked about it before when I was testing a server and was told it's a feature they don't have plans to enable. The only way to currently test any subdomain pointing at your IP, or even your IP, is through a VPN or a separate connection than your home one, like cellular as you've already found out.
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