Can I keep my old plan on 5G?

I currently have an iPhone XS and am still on the T-Mobile ONE with ONE Plus plan. I’m planning on buying the new iPhone 12 mini soon. 

Will I need to switch to a new plan? Since the new iPhones are 5G compatible, I wasn’t sure if I would need to switch to a Magenta plan to be able to access 5G.

Thanks!