Mine has the option under advanced to either 2.4, 5.0 or auto setting. Phone came from verizon tho, so probably different than the unlocked. Yes most routers have both 2.4 and 5.0 and modern phones such as the 10 have the ability to use either and will just connect automatically.
I was having trouble holding on to wifi when further away from the router so checking my settings I saw it set to auto. Changed it to 2.4 and no more issue at all. Kind of along the same train of thought why the default cellular setting is auto to global but never CDMA/LTE. I always get better , more stable RF when switched to CDMA/LTE and is one of the first settings I change when setting up a new phone. Why default is auto? No clue but the man in charge of such things doesn't necessarily know. Same with the wifi setting apparently. Why Auto? Glad some stuff is adjustable.