Trying to install Jitsi on backend server - issues

I am trying to install Jitsi Meet on my works server to allow video conferencing.
I have managed to get it to work if using a Self Signed certificate and signing in to the LAN using a VPN connection.
But if I try to get a Lets Encrypt certificate using a domain setup
“sub.company.ex” A record to the routers public IP
I cannot install a certificate.
Strangely, if I use a free DNS Server (Afraid DNS) using the Domain “company.chickenkiller.ex”, I am able to install the certificate on the host.computer machine.
I believe I also need to setup a reverse proxy on the main webserver to redirect “sub.company.ex” to the host machine but it seems odd that using the “company.chickenkiller.ex” domain works but “sub.company.ex” does. (Although this may just need to propagate.
Once this has propagated, will I be able to install the lets encrypt certificate?
At this point, is there a simple guide to setting up a reverse proxy in Apache to redirect traffic from “sub.company.ex” to host.computer