@Roman-7880, To add second ADFS server, you just need to install ADFS Role and add the new server to existing ADFS farm.
Should I reconfigure sync in "azure ad connect"?
No, you don't need to reconfigure AD Connect. However, if you have only one AD Connect server in your environment, you may consider installing AD Connect on the new server and keep it in Staging mode. In Staging mode, AD Connect receives all inbound updates (imports) but doesn't export anything. So, in case if the primary AD Connect goes down, you can turn off the staging mode and use the server as production AD Connect server.
Is that possible, to use cluster adfs without wap?
Yes, you can configure ADFS Cluster without WAP in place. Although, from security perspective it is good to have WAP installed on a non-domain joined computer as it is installed on internet facing machine. With no WAP in place, you will have ADFS Servers facing internet and in case of compromise, malicious user will get access to the domain.
Is it possible to use dns round robin?
Yes, you can use DNS round robin but I always prefer NLB over DNS round robin. The reason is, if one server goes down, DNS doesn't have intelligence to detect that and it will keep resolving every second request to the server which is down. As a result 50% requests will go to the faulty node. However, NLB can detect faulty node and will send request only to the node which is up and running.
Hope I have covered all your questions.
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