Unlikely, 1) Keep in mind in the US Part 15 users are secondary to any and all licensed users of the 2.4ghz band This means the hardware has already been designed to handle use in environments where it may not have control of the band 2) For 21 miles you likely will be using high gain dish antennas, this means the beam path is very tight and that signals from your node will very quickly fall into the -60dbm gain range of the antenna being virtually eliminated at the time it leaves the antenna, after that house construction will also attenuate the signal as well. By the time you get indoors your node is but a speck of noise compared to the internal RF reception of all the homes own devices. 3) WIFI is a Carrier Sense Multiple Access protocol. They are more likely to cause you trouble with corrupting some of your packets because they will in no way be able hear your node 21 miles away meaning they will think the channel is clear (hidden transmitter syndrome) 4) Setups like ours have been around for a long time under Commercial Wireless Internet Service Providers. How often do you hear someone complain they can't get WIFI because of them? Heck houses even use a WISP connection and than run WIFI in the same house some times and not report issues. Household connections need to worry about the Microwave Oven (which really wipes out the band indoors) more than they need to worry about us outdoors.
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