Welcome to the MongoDB Community Forums @Santosh_Dhaukonda!
You are already on the right track: if you want to isolate readonly queries with different usage patterns so they don’t affect your application performance, I recommend adding a Hidden Secondary to your replica set and setting appropriate firewall rules to limit your end users so they can only connect to this secondary. The connection string format to use in clients like Robo3T will be the standalone / direct connection rather than a replica set connection string.
Clients will not distribute reads to the hidden secondaries, so the only traffic they receive will be for replication and via direct connection.
I would also recommend using a more actively maintained admin client that uses a supported MongoDB driver. Robo3T embeds a specific version of the
mongo shell (currently 4.2 as of Robo3T 1.4) and this may lead to unexpected outcomes. MongoDB Compass is the officially supported (and free) GUI, but there are commercial alternatives like Studio 3T which may have other features of interest depending on your use case.