Unable to connect to server with online mode

I have recently converted an old PC into a full time Minecraft server, and installed the vanilla 1.15.2 software, I installed Ubuntu server 20.04 LTS on it and activated the server with the default server.properties file. I then went to see if I could connect and I was greeted with an error “Authentication servers are down- sorry”. I then restarted the server with online mode set to “false” and I was able to connect, however this setup doesn’t work for me. I have tried sudo update-ca-certificate and sudo ufw disable but it changed nothing. Please respond to this as soon as possible.