When a connection string starts with
mongodb+srv, It indicates the connection uses DNS records. This seed list allows us to abstract all nodes in one replica set member. However, there are still a number of hosts running underlying the abstract. That is why nslookup will not work against this seedlist connection string which has no real IP address. In Atlas, by default there are three actual hosts behind this string. We could try to access one of them.
$ nslookup mflix-shard-00-00-cbfye.mongodb.net
mflix-shard-00-00-cbfye.mongodb.net canonical name = mtm-aws-apsouth1-0-m0-6-shard-00-00-v3k13.mongodb.net.
mtm-aws-apsouth1-0-m0-6-shard-00-00-v3k13.mongodb.net canonical name = ec2-13-127-65-58.ap-south-1.compute.amazonaws.com.
If you want to use nslookup specifically with srv, you could try
[root@qi-docker01 ~]# nslookup
> set type=SRV
_mongodb._tcp.mflix-avw5t.mongodb.net service = 0 0 27017 mflix-shard-00-00-avw5t.mongodb.net.
_mongodb._tcp.mflix-avw5t.mongodb.net service = 0 0 27017 mflix-shard-00-01-avw5t.mongodb.net.
_mongodb._tcp.mflix-avw5t.mongodb.net service = 0 0 27017 mflix-shard-00-02-avw5t.mongodb.net.
I think you could try to ping one of replica set member. If it fails, maybe the firewall setting is the cause.