I have just got a brand new Pixel 3 from EE with the same sim I had before in my old phone (which was EE).
It seems everytime I connect to a Wifi signal, after I disconnect from it my 4G does not connect to the internet correctly. I get full signal and 4g status on my bar but when trying to load "internet intensive" apps like youtube or twitch it will not load. Sometimes I can use WhatsApp and FB Messenger but others not.
The APN i am connected to is EE Internet (everywhere), I never had this problem with my last phone which was not locked to a network.