This may have nothing to do with Google Fi. My wife and I have the Google Fi plans on our phones and get our regular messaging app texts just fine. I got a data only sim and put it in my older Moto G5+ phone that I gave to my son so he can use WhatsApp to message us from the town pool or where ever else is needed.
The problem is that I sometimes don’t get his messages for 30 minutes after he sends it and visa versa. I know that WhatsApp doesn’t use SMS so its unrelated to the messenger app which works fine, but any ideas why Whatsapp messages take so long? They do get delivered quickly when he is on WiFi. Data Saver is not on and WhatsApp background data usage is enabled.