I have two servers, one in OVH, and the other in Hetzner.
For my domain, in the domain control panel, I have listed two name servers, each pointing to one of the servers, each with an active DNS resolver. Both DNS resolvers report both IPs for the A records (the main domain and the www subomain/version). The TTL is set to as default (14400). Also, both servers report both nameservers consistently (the NS records are the same) and the SOA records of both DNS resolvers are identical, so I got no errors or warnings when I checked it by dnscheck.pingdom.com.
My website is rarely updated (static somehow), so I don't need software to synchronize the data on the servers. It is running on HTTP; no SSL/TLS.
Now, my main question is:
Will every user be able to connect to the site if one server goes down suddenly?
And suppose the servers are not going to be changed:
What if I set a long or short TTL: 864000 or 3600?
I would add that this is only to ensure that my website is available; it is not for round-rubin DNS, because my server load is quite low.