How to control setReadPref on mongo sharding

Dear all,

I have deployed mongo sharding with 3 shard , each shard is a replicaset (1 pri +1 sec + 1 arb) version 4.2. All application connect throught mongos instance . When I monitor by netdata , I see all traffic read and write go to primary instance , how to change read to secondary and write to primary .
Notice that mongos isntance using auto deployment tool some thing like auto scaling group so how to apply this pattern read + write above when mongos auto start

Thanks you