The basic instructions can be found at https://www.axigen.com/documentation/managing-ssl-certificates-p21594182.
For me, I was having issues because Axigen needs to be able to respond on port 80 for the Let’s Encrypt system to confirm you are who you say you are. I was having difficulties enabling non-SSL Listener, so I changed Axigen’s non-SSL Listener’s port to 8080 and routed all port 80 traffic to it via my router. Once I did that, I was able to quickly get the Let’s Encrypt cert and bind it to all of the listeners. You can read about that in my posts here - Let's Encrypt Certificate