The short answer
Use the exact phone model and carrier certification, not a generic 5G label, to determine whether a device can use mid-band 5G.
Step by step
Locate the full model number in Settings or on the original receipt; regional versions of the same phone can support different bands.
Check the manufacturer's technical specifications for the mid-band frequencies used by the network you are considering.
Run the carrier's IMEI checker because supported hardware may still need carrier software or certification.
Install current operating-system and carrier-settings updates before testing service.
Test outdoors and indoors in places you regularly use the phone; a 5G icon alone does not prove C-Band performance.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.