100% it is the BT router's fault.
I have 2 routers at home, the BT one where the broadband routes in, and then another which I connect to the BT router via cable. I can get Whatsapp and Slack just fine on the second router (which is connected to the internet via the BT router).. so the Whatsap and Slack(chat) services are coming into the BT router, but its not being accepted over the BT router's Wifi ariwaves... I hope BT pickup my comment and give us an answer.
Make sure you have set the web pages as acceptable in your security settings go back in this string to see what you need to do and what web pages you need to include. This may work
I think I fixed this issue ages ago by changing the channel to a less congested one. Some Samsung phones do have delayed notifications. But it seems to have been fixed with the latest updates