Ah. I’m not sure on that specific setup sorry, but sounds like you may need to check what chains already exist and see if there are ones for the right network interfaces. I’m familiar with IPTables itself only, not the Asus openwrt!

I’m not familiar with that OS in particular, but yes, IPTables can restrict egress to certain IP space very simply.

They charge this because people pay it. Maximise your ROI as far as you can get away with is a staple of many industries.