A router is in simple terms just a radio and needs certain receivers to receive different channels same as a tv to change channels your actually changing frequencies.
You have a 2 channel router that transmits at two different frequencies so you need receiver’s that receives two different frequencies...
I think he is confusing frequency and bandwidth. You don’t “get more flow” from using a 5 ghz device instead of a 2.4 ghz device... they are just different ranges.
Your bandwidth and upload / download is determined by your ISP and the equipment between you and the nearest switch.