Can't join domian within 2 sites share the same domain

Question

There are 2 sites with the same domain, abc.com, the subnets are 192.168.0.0/24 and 170.20.10.0/24, they can't access other except the 4 domain controller, the domain data are synced via bridge domain controller, but there's issue when try to join the domain, the 4 DC will work via round-robin, so it may fail when try to join in domain or log in.

Any suggestion or idea is high appreciated. Thanks,

The AD user and DFS is used to share in both sites.