Your best bet is probably PostmarketOS, its not debian but alpine, but does support the phone and aparrently still gets some updates.

Theres a link on the page to the non T variant with flashing instructions.

https://wiki.postmarketos.org/wiki/OnePlus_5T_(oneplus-dumpling)

It should work as long as it can reach the internet

You can’t expose ports on mobile data. You can ether host the web server at home or on a vps or use something like tailscale.