When multiple devices are connecting to your broadband service, the available bandwidth of 1Mbps is shared between them; that cannot alter whatever router you use.
Again, when multiple devices connect "wirelessly" to your router, the available bandwidth of (presumably) 54Mbps is shared between them, unlike "wired" ethernet connections where each device will have the full 100Mbps over the network. However, this is fairly academic as the shared portion of 54Mbps will still be well above your 1Mbps broadband.
As far as dual band WiFi is concerned, some routers use either 2.4GHz or 5Ghz, others can support both simultaneously. However, the Wireless Network Adapters in your devices also need to support 5GHz to use this option.
You could do a test by connecting the Laptop that is hogging resources by ethernet cable instead of wirelessly, to see if that evens out the bandwidth.