It looks like that by changing the
0 on the channel mask ALL the 8 500kHz channels - which appear on 1.6Mhz boundaries - became enabled. Effectively the same as changing the 1 to FF.
(500 kHz channels 64 - 71 are at:
903, 904.6, 906.2, 907.8, 909.4, 911.0, 912.6, 914.2
which matches what you're seeing)
So I'm really not sure how to disable these higher channels. But I'm surprised if your gateway isn't listening on one of those 500 kHz channels too, is there some configuration? I'm not sure how Microchip gateways work.
Also are you running the latest Laird firmware? You can check your version with
at i 3
latest for the RM191 should be 220.127.116.11