Remote Desktop Services - Users prompted twice when launching remote App

I am currently testing RDS in our environment on 2019 servers. I followed this guide to set this up.

https://www.anotherwindowsblog.com/2018/02/building-redundant-microsoft-2016-rds-farm-netscaler-part-1.html

I applied a wildcard certificate for all the services. 

When the user logs on to https://rds.mycompany.com/rdweb with domain credentials he is prompted for another login to the RD Gateway. 

The RDweb service is running on the same host as the RD Gateway. I have come across applying group policy, trusted sites etc. But my issue here is the users will be logging from a non-domain joined home PCs. Please help.