Subdomain with HTTPS redirects to Main Domain with HTTPS

hai

I add a subdomain with my main domain.
I already added https with my main domain and now i added a sub domain. The problem is if i open Sub domain like https://www.sub.maindomain.com it works fine but if i open sub domain like https://www.sub.maindomain.com the url remains same but it redirects to main domain content.

If i install any plugin in Sub domain to redirect from http to https suddenly the sub domain redirects to main domain so, please give a correct resources to fix this problem.