Set different Upstream for specific source IP

Installing and Using OpenWrt Network and Wireless Configuration
2

You need to do Policy Based Routing and you have 3 options:

  1. mwan3 package
  2. pbr package
  3. a set of rules/routes for each internet connection.

In your case you have not filled in the rule properly. You need to specify Incoming Interface, Source IP, and the Table. Then in static IPv4 Routes you need to add a default route and any local routes for that Table.