This is the reason why I went out of my way to use Molly (a fork of Signal), since it supports delivering the push notifications through a self-hosted server instead. Unfortunately the process is complex: it requires both a method to deliver the notifications to your phone via UnifiedPush (an alternative to Google's push system that generally suffices on its own) and a compatibility service called MollySocket (that bridges Signal's notifications with the UnifiedPush provider). Both typically need a self-hosted server and specific configuration to talk to each other though. And I don't even have any contacts that use Signal anymore, so, well...!