Was wondering if you need to use a phone that has service to fly my Mavic air. Wanted to upgrade my samsung S8 for an S9 and use ny S8 exclusivly for my drone.

A cell with no SIM card for phone service still works just fine to run Go 4 with your MA.   Just use the phones wifi to do the updates for Go 4.    I used an old Note 3 phone before I got the CS.
Of course you will not have live maps but offline maps will work.

You definitely don't need cell service to fly. Many people, including me, use a tablet with no service other than wifi. I use a Galaxy Tab S2 in airplane mode.

We run our drones on an S5-S6-S7 with no sim cards, no issues.

Generally, it is recommended to enable the airplane mode to avoid the interference. No worries.

Thanks for the responses. Moving forward.