I don't even know if this will be picked up by anyone willing to investigate and help but it beats talking to a brick wall, otherwise known as Virgin Mobile customer services.
Since I moved away from EE to Virgin in September 2019 I have had the same issue of delayed text messages/iMessages/WhatsApp notifications and recently I discovered when this happens that my Find My iPhone service also appears offline meaning my location cannot be found (this has caused a great deal of concern for me as I cycle on roads and in the event of an accident my wife would not be able to find me).
I have attempted numerous times to speak to customer services, a new SIM has been sent which has not fixed the issue. Apple have fully checked the phone hardware and software assuring me that this issue lies with Virgin Mobile. I have tried to explain this to advisers but to no avail, I then requested to speak to a manager which has yet to materialise despite numerous promises to call me back (first time before 6pm that day, then a manager did take my call briefly when I called customer services again but was incredibly rude saying he would need to phone me back as he was speaking to another customer, even though I offered to hold and wait for him. The call back time frame was then moved to 72 hours which has now been and gone).
I tried raising a complaint online, which stated it would be confirmed as received within 48 hours which again has now been and gone.
I would like to speak to someone who can actually do something about this, I am not being provided a full reliable service yet I am expected to continue paying. I have been told to cancel my contract I will need to pay £325 for the privilege.
I want to cancel my 30 day rolling contract and also the handset loan, without any penalty, as I need to move to a more reliable network.
I will post the handset back to Virgin.
Here's hoping someone picks this up and actually does something about it.
I'm sorry to hear that you're having problems with mobile data when using certain Apps. Using the information we have on here I've found your details and have checked for any local outages and the masts and can confirm that there are no issues with them.
I can see that you have been able to contact us since your post and we have managed to update you on the situation and your complaint. We can see that this is being dealt with as it should be and we hope that we are able to reach a resolution with you soon.