Android Go - Notifications missing

My daughter has a low specced Android Go phone, just for Whatsapp, Signal and txting. She likes it as it's also quite small compared to most other smartphones.

I don't mind it's not fast, but I've noticed that notifications for Signal & Whatsapp are not coming through after being idle for some time.

I've disabled battery saver for these apps, but after a couple of hours, notifications are not coming through again. What else could I change?