If you are having LTE issues with your Pixel 4 and Fido and Google won't and can't help you see the steps below that I've worked so far. it looks like these phones were never set up to work on the Rogers networks properly. Also some US carriers. I don't fully understand it but this was a post on the Google forms that has worked so far.
1. Enter testing menu by entering *#*#4636#*#* on phone
2. Select 'Phone information
3. Press the 3 dots at the top right of the screen and choose Select radio band'
4. Choose 'United States, you should get a confirmation message.
I have been running rock solid now for 3 days and haven't dropped data once. Before I had to leave my phone on 3G so it would work. Now it works all the time on LTE.