That’s what I hoped - and that’s how it works with Molly (Signal FOSS with UnifiedPush)
When I run troubleshoot notifications on Element X it shows the test push gateway as matrix.gateway.unifiedpush.org
It only errors on the final ‘Test push loop back’ which it says ‘pusher has rejected the request’
I whitelisted the IP of the NextPush server too
Edit: Not sure what happened but this suddently started working on its own