Self-Hosted Main

If you have IPv6 and don't have a firewall you could have acces directly to your server.

Use a P2P VPN like ZeroTier. I use it even when I have router access because I'm hyper paranoid about exposing anything to the open Internet.

If you cannot forward a port in your router, then you need to run a tunnel to the outside. Look at VPNs, like Tailscale/Headscale, plain Wireguard and Cloudflare tunnels etc.

Use an outbound tunnel like the others recommended already.

For curiosity: a mobile hotspot in my mind is a device connecting to Internet via mobile data (LTE, 5G etc) and offering a WLAN. In this case this is your router. But mobile provider often offer no public reachability at all - in which case you are back to the tunnel.

This is my policy:

For publicly accessible services like a website, I use a cloudflare tunnel.

For restricted access to just a few users, I use a cloudflare tunnel and a cloudflare application to manage access authentication.

For my exclusive restricted access to the infrastructure, I used tailscale.

Zerotier, or any other VPN service that is outbound/cloud hosted.

This question has been answered many times now with multiple solutions, in fact so many times that Google has decided to dedicate the whole search engine to just display solutions for this very common question.