This doesn’t necessarily mean that the phone is losing connection to the mobile network. If you see that the reception icon is empty, or has a cross in it, then it could be if that state persists for a long time.
I’m assuming you’re referring to WhatsApp/Signal messages, rather than MMS.
Have you tested whether cellular data works? (e.g. can you visit websites away from your wifi?)
If it does, have you tried opening the app when you’re away? If you do get the messages away from wifi when you open the app, then background data is probably restricted for that or all apps (see here).