Since your examples suggest that the services you plan to deploy require SSL certificates, the first thing you should do is to buy a domain name from a domain registrar. Therefore, you cannot use AirVPN's DDNS to apply for SSL certificates, since you do not have ownership of airdns.org. Assuming that you have bought a domain name, these are the next steps:
1. Go to AirVPN client area.
2. Choose sessions.
3. Take note of the Exit iPv4 of each of the sessions that you have connected to your PFSense Router.
4. Go to your domain registrar, and register an A record for each ipv4 exit IP address from step 3, and register an AAAA record for each exit IPv6 from step 3.
5. Take the DNS-01 challenge from Let's encrypt to get your SSL certificates