I can't modify the router config and I want to access my server from internet
I have a server in my home and use it normally with my custom router setup. but I want to do the same in other place where I'm not allowed to modify the main router
How can I expose a port to internet in this case?
P.D: I want to use my laptop as a portable server; connect it to my mobile hotspot and go, I think that the solution will be the same for both cases
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 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.