Messengers don't show incoming call when they are not active

Samsung A10 with Android 11. Nor Viber, nor Telegram shows incoming call when they are not active. I mean, that if the messenger is open (in memory) and it is currently being used (focused) then incoming call is shown. However, if another application is used (but messenger is still open) then incoming call is not shown. I think, the problem is that when messenger is not active it loses internet connection, because when Telegram becomes active (user switched to Telegram) it shows in the title "Connecting". Viber also shows something similar. This is my mother's phone. It worked well before, but something happened. Either she changed some settings or the problem in updates. Could anyone help to fix this problem?