Couldn't login to admin panel after disconeccting to Internet

I installed bitwarden on my server for using in our company, I used self-signed certificate, and used owned mail server, it worked correctly, but after doing tests, I disconnected this server to Internet, the serve has access to mail server, and mail server has access to Internet.
But I recognized that I couldn’t login to admin panel anymore, why? I still can login to user’s panel.