Can Google Photos on iOS use mobile data connection to back up?
Back up on mobile data: If you don’t want to wait for Wi-Fi to back up your photos, under “When to back up”, tap Use cellular data to back up photos or Use cellular data to back up videos. Charges from your service provider may apply if you upload using your mobile network.


from https://support.google.com/photos/answer/6193313?hl=en&amp;ref_topic=6156061&amp;co=GENIE.Platform%3DiOS&amp;oco=1

Also, turn off low power mode for background backup

As OP puts it nicely: 


  ...the key is: you have to allow iOS to allow mobile data, and allow the Google Photos apps to use mobile data, so there are 2 settings to be touched. [..] we can have on/on, off/off, on/off, and off/on, and only on/on allows using mobile data...


From: Can Google Photos on iOS use mobile data connection to back up?