I think you already check the existing post, Setting search_path for Postgres Resources, which shows the connection settings doesn't work. Actually the set search_path could work in different levels, that post solution is the query level. You could use the user or database level like below:
# user level
ALTER USER your_username SET search_path TO schema1, public;
# database level
ALTER DATABASE your_database_name SET search_path TO schema1, schema2, public;
Of course you have to run this query out of retool, such as the psql. Then you can use the normal retool connection settings.