Independence of the domain name from the provider. Fault tolerance?

Question

Computer networks

Hello, I don’t know how to formulate exactly, if anything, please clarify ...
The apartment has Internet from two providers, on reg.ru there is a VPS where dns and hosting of the 1st site are configured.
you need to organize the following:
create a domain name like 3rdlevel.mydomain.com when accessing which I could connect to the VNC server on a non-standard port, there are no problems here, you can add an A record with the provider's external IP.
BUT when ISP 1 fails, I want the domain name to be accessible by the IP of ISP 2.

How can this be done and which router is suitable for this? at the moment I use pfSense to

balance or distribute the load, there is no need, but there is a desire for the chain to work constantly:
3rdlevel.mydomain.com:15999 -> wan1, and if wan2-> lan -> vncserver is unavailable,
respectively, if wan2 is unavailable, switching to wan1 (well, or using some script to monitor the availability of wan1 and return to it) ....

but I guess that I I don't know something and I want to understand it ...

Answer 1

Connect the dyndns service to the router. If wan1 is down, then wan2 will be. The domain name will be updated to the new IP as well.
You will always log in by the name of the 3rd level domain, but in the dyndns zone.