There may be an App for your phone that can show you the different wireless signals and from that it may be easy to decide a channel which isn't in use, or a gap between two other 'strong' signals from neighbours.
From what I've seen, many routers automatically use channels 1 , 6 , or 11 (Sometimes a router generates several signals on the same channel, such as a BT router, one for the customer, and a public signal for other BT Internet users as part of the 'FON' wireless service.)
I took a snapshot from my mobile and have posted it. Normally there are signals from someone using BT, someone else using Sky on channel 1, and someone using TalkTalk on channel 6. I therefore set up my router (BWlan) on channel 4, as I didn't want it to be swamped by the other signals (I am upstairs at the back, while the router is downstairs, at the front of the house).
Being able to see (or get a list of channel numbers in use) may help you sort it out in 5 minutes. You'd need to get into the command menu for the rouer and change the wireless networking from 'automatic' to 'manual', and then decide the channel number based on which channels are being used.