DSL as I recall deteriorates over distance from the main line. As people closer use the signal it gets slower. I could be wrong, but that was what I remember about it.

Here are 2 articles about this (old by the way):

http://www.onlinebusinesshome.com/is-distance-a-problem-for-dsl-internet-aid-16281/

http://www.plugthingsin.com/internet/connection/dsl-vs-cable-internet/

Of course this is not the case if the connection uses fiber to deliver the signal.

As for changing bands, well, the 2.4 and 5Ghz signal strenghts are different. Closer to the router, the 5Ghz remains stronger until the far end of its strenght. It drops off in strenght dramatically (distance which this happens depends on many factors, and includes home wall contruction materials and number of walls between the device and router. However the 2.4Ghz signal degrades faster but goes further, and at some point will be the stronger signal. Devices that AUTO CONNECT can sometimes switch SSID's when this happens. All devices you DO NOT want to use the 2.4Ghz SSID should have that network 'forgotten' in the device network settings. However not all devices might have this feature.

Some reading for you:

https://support.stuff-fibre.co.nz/hc/en-us/articles/226438147-What-is-the-difference-between-2-4-GHz-and-5-GHz-

https://www.howtogeek.com/222249/whats-the-difference-between-2.4-ghz-and-5-ghz-wi-fi-and-which-should-you-use/

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Is the CM1100 compatible with Dlink DIR-655