"This network is operating on Wi-Fi channels in use by several other networks" problem/solution

Have you tried turning off the Router for five minutes and plugging it back in? Have you tried resetting your network settings? Settings>General>Transfer or Reset iPhone>Reset>Reset Network Settings.

Check your router manual; it should have a way to search for the least used channel. Usually just turning it off for a few seconds will start a search, as IdrisSeabright suggested.

Wi-Fi can support multiple users on the same channel because it is a frequency hopping protocol, so a bunch of users, each using different random frequency changes, can share the same bandwidth (technology invented and patented by the actress Hedy Lamar, who was an engineer before becoming a performer).