ADSL is generally not a fixed IP, which means that the server does not recognize it.
As I understand it, you want to balance the load.
Analyzing your doubt. I leave my case study.
1 - How each server is separate.
Place a fixed IPv4 from there, create a fixed DNS and point to the corresponding fixed IP. Also place a domain and add the DNS configuration. If you have IPv6 (AAA), add it for even more answers. And prioritize cryptography.
If you need a secure proxy, use CloudFlaure to manage your settings. It helps the configuration become more practical.
2 - Pointing the DNS to fixed IP, create NS for both servers.
example: ns1.server.com //// ns2.server.com
I recommend creating 4 NS for each server.
3 - The same procedure is used on the slave. but do not use ns, switch to another silga, for example: (tl1.server.com).
This facilitates orientation.
So, for each server.
An IPv4
An Ipv6
Four NS
Two DNS
Therefore, when the first server goes down, the other server takes over. Be on IPV4 or IPV6. And the DNS and the DNS are congested or are not recognized for some reason, promptly the other takes over. So, without loss.
If there is a problem with your hosting, create IP or domains. Due to fees.
There are some free ones and, that work 90%, cloudflare itself has this service.
If I was able to understand your demand, I recommend the services later.
It works for me.
Submit questions and follow the case.