Some random thoughts 
In scenario 2 you want to WISP connect to your main router on one band (e.g. 2.4Ghz) and connect your devices to the other band (5Ghz) or vice versa. This “maximises” bandwidth not using a “shared” band.
Other thing to do is to get something like the Ubiquiti WiFi man on your mobile/cell and have a look at what other WiFi networks are around you and MANUALLY set your band to the best “non-competing” channel. Note that you won’t be able to set the channel for your main router connection band as it will have to be the same channel that the main router is transmitting. Might be worth looking at and potentially changing that channel as well.
Also make sure that the adapter your using is putting out at least 5V/2A. Make sure it’s a good quality power cable between the AC adapter and Slate.
As mentioned above I’d try Ethernet connecting first and see what sort of speeds you’re getting off both 2.4 and 5.
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