ORBI5000 network

The advantage of using 40 MHz channels is the transfer speed on 2.4 GHz is increased by about 2X. The downside is almost no ability to avoid interference from/with neighboring APs. The interference will reduce your throughput, so the benefit could be theoretical only, unless your house is a cabin in the woods. If throughput is critical, then 5 GHz devices should be used anyway.

The recommendation from the pros is to only use 20 MHz on 2.4 GHz and to manually set the channel to 1, 6, or 11. Don't use Auto channel settings.  You could agree with your neighbor to use channel 1 while he uses 6, or something similar. You both would benefit.

Some more information that might help you:

Orbi will always try to operate with a 40 MHz wide channel in the 2.4 GHz band. If the coexistence setting is ticked, then Orbi will fallback to 20 MHz if interference from a neighboring AP is detected.  So, your neighbor cannot explicitly set a 20 MHz channel width.

The only way to operate inteference free with 40 MHz is with one router on channel 1 and the other on channel 11, and one of the routers using 20 Mhz width. 

If your neighbor sets channel 1 and you set channel 6, he would fall back to 20 MHz width.  But, if the neighbor sets channel 1 and you set channel 11 (or vice versa), then he could continue to operate at 40 MHz and neither of you would interfere with each other.