Connectivity between a modern home router and an iPhone 7?

I am assisting a friend who has lost a phone and is considering getting an older phone (iPhone 7) as replacement, as it is well regarded in the digital photography profession. The concern is that the router at their place, latest Comcast product, may produce issues with a legacy phone, such as the iPhone 7. Are there any known issues with a latest model Comcast router and older Apple iPhones? How old of a phone model could they go? iPhone 6 or 5, perhaps? They are concerned that the router may combine both 2.4 GHz and 5 GHz signals that the phone may have difficult connecting because it can't distinguish between both signal types.