Need help setting up VPN for Octoprint and Home Assistant on two separate machines

Hey guys, I'm trying to setup a VPN that allows me to tunnel into my Octoprint and Docker-based Home Assistant instances which one on a Raspberry Pi 3 and 4 4gb respectively and have each instance accessible via a DuckDNS DDNS address. So far I got the VPN up and working but I have no clue on how to get the DDNS working for Octoprint or Home Assistant. Particuarally, The DDNS for Octoprint keeps throwing this error:

Rejected request from RFC1918 IP to public server address

Along with these errors in the System logs

 164015       : Registered IP '' detected
 164015  info : Rerun IP check at 2020-12-31 16:40
 164015       : Detect local IP on 'network'
 164015       : Local IP '' detected on network 'wan'
 164015       : Waiting 600 seconds (Check Interval)

And as for the Home Assistant DDNS, I haven't actually setup yet but I'm wondering how do I configure it since it's in a Docker/Portainer container within OpenMediaVault 5 and to access it I have to specify the port for it (which is the default one for Home Assistant instances, 8123).

For setting up the VPN and the DDNS for Octoprint, I followed a guide I created back in June, which is based on these: VPN Server, DuckDNS, VPN Extras. The original links I had in the guide are gone now.