Problems to configure SSL / HTTPS to my gitea instance

Hi I am new here and quite a beginner so sorry for the stupid questions.

I have gitea up and running and it works great. Currently it is only delivered via HTTP
but I would like gitea to be delivered via HTTPS. My server has no own domain and is only accessed via IP.

Additionally I have a website on another server where a subdomain is set up ( I want to make gitea accessible under this (Sub-)URL and that via HTTPS but I don’t know how. Do you know a step by step tutorial that could help me?
I have already tried a few but none of them helped me.

Thanks in advance