I've been experiencing the same issues since day one of using Virgin Mobile. I use Whatsapp desktop and whenever my phone has been locked for a few minutes the connection to Whatsapp drops, I then have to unlock the phone and go into Whatsapp to sync.
I have reported this to Virgin, had a replacement SIM, performed a factory reset on the phone and had no luck. This is only on mobile data, if I'm connected to WiFi then I get push messages instantly.
Fortunately my contract ends in the next week so although I'm gutted to lose out on a cheap 20GB data plan it's useless when I don't get the full value from it.
I have a Huawei p20 pro and have the exact same issue, it's driving me crazy!! It's only on 4G, when I'm on WiFi they come through fine. I can be out and about on 4G and get nothing, only to open whatsapp and have messages there from hours before. Very frustrating, particularly when I'm only a couple of months into a year's contract and would like to leave if there is no solution!!
Good day! Due to our current system upgrade, some unforeseen mobile disruption has occurred. In effect, we’ve been receiving a high volume of messages. We greatly appreciate your patience and understanding whilst we work diligently on improving your mobile services. Rest assured that our network engineers are already working on it for you so you can continue enjoying our service.