I live in a row of 5 attached 2-family townhouses. I estimate that each family has a dual-band router which broadcasts at least 10 different SSIDs. At any given moment Net Analyzer app shows about 12 to 15 WiFi signals on both 2.4 and 5 GHz.

I have heavy metal fire proof doors, refrigerator, and washer and dryer in the center of the house resulting poor reception from one side to the other.  I subscribe to 200 Mbps from my ISP but transfers rarely exceed 50 Mbps of WiFi. To resolve this issue I have installed (wired) tp-link RE220 extender working as a access point on the north side of the house and the A20 router on the south side.

I suspected interferences from other WiFi signals were to blame so I changed channel on my extender 5G signal to unused channel 100. Immediately speeds reached 175 Mbps. I tried to use such tactic for the main router (A4000), however the router has way less channels available.

Extender channels available: 36, 40, 44, 48, 52, 56, 60, 64, 100, 104, 108, 112, 132, 136, 140, 149, 153, 157, 161, 165.

Router channels available on band 1: 36, 40, 44, 48

Router channels available on band 2: 149, 153, 157, 161, 165

All these channels are very crowded. 

My question is what happened to all those channels in between? I assume that setting any of those missing channels would solve my interference problems.

Is there any other way to resolve my problems without investing in $1000 Netgear Orbi Pro or something similar?