How do I fix delayed WhatsApp notifications on iPhone 12 after broadband change?

Hi,

I'm using an iPhone 12, and ever since I changed broadband providers about ten days ago, the majority of my WhatsApp notifications are delayed in arriving, usually between 10 - 30 mins after being sent.

I have done literally everything it says to do. Switched on Background App Refresh, reset notifications, made sure all my notifications are on, checked for new updates on WhatsApp and the phone itself (all are up to date; I installed the recent WhatsApp update on Thursday, and my phone is running iOS 18.3.1), uninstalled and reinstalled WhatsApp (with my data backed up) and even reset my phone settings back to their original state, to either no avail, or for a few to come through on time before the notifications become delayed again. When I've got the app open however, there's no issue whatsoever.

If I'm outside or in another place with WiFi, this issue doesn't appear to occur. I tend not to have my 4G on as it eats into my credit otherwise (I'm ancient and still on a pay-as-you-go model). The signal on my phone appears to be much weaker too since changing over, lingering around one or two bars. We've got hold of our Internet provider, who has ran tests and sent us a booster, which we're using, yet still the issue continues.

Some WhatsApp notifications arrive within seconds of them being sent to me, but others take forever to arrive, and it's driving me insane, because I just can't seem to figure out what the problem is. Is it a bug? An issue with the phone? Any help would be greatly appreciated.

[Re-Titled by Moderator]