It doesn’t work that way, and none of that even matters if you are using your router as a VPN router, as your external connection from a cafe is slower round-trip than any local devices to each other.
Dual-band wireless routers are only useful for speeding up the data throughput of locally connected devices, and then only up to the maximum throughput of the two devices connected to each other, with the limiting factor being the slower device.
That said, if you have a “slow” device at 54 Mbps, and three devices at 100+Mbps, the slow device only slows down communications when another directly talks with it, not when the other devices to the others.
Often, the dual-band wireless feature isn’t fully utilized, as the fallback in congested neighborhoods is to instead migrate communication to the least used bands, even if it means you aren’t using a bonded, dual-band setup under Wireless N.
You may be right, but that is not the understanding I got from this article: How To Buy a Wireless Router: The Short Version - Speed, Choose Type, Products
I would still spend a little more and go with the dual-band, dual radio model.