Ok, So I'm going shopping for an iPhone on Friday, But the thing is, My dad is buying the phone and my grandad is paying for the service, but that means my dad has to buy the phone without a carrier, but he doesn't want to pay for the phone all at once so is it possible for him to do like 20$ a month even though he's not buying the service?
Your dad can give your granddad the monthly price, whatever it may be, and put it on a payment plan.
Unless Apple does their own financing somehow (I don't know Apple's financing options).
T-Mobile will not let anyone pay for a phone in installments without a plan (no carrier will do this). However, Google allowed me to purchase a Pixel2 through their financing option instead of through T-Mobile's. If Apple has similar, that's feasible.
Manage your T-Mobile account