@ljutica
It's a bit a workaround to get notifications on signal, but it goes like this:
Get UT Tweak Tool:
https://open-store.io/app/ut-tweak-tool.sverzegnassi
Open UT Tweak Tool -> Input Password -> press the menu -> app scopes -> Signal -> Turn on "Prevent app suspension"
Open Signal, in each conversation for which you want notifications you open the conversation -> menu - > turn on notifications
leave the signal app open and running the background and you should receive notifications when messages come in.
It's definitely got some quirks...