I assume you have already done a proper Forum Search for your model number in case other people have already answered your question ?
If so and that didn't help then check out this thread: https://community.sling.com/message/10363#10363
Come back if it that doesn't help either.
I am having the exact same issue, I have an Arris Moxi set top box with MP2000NA controller. Everything was working just fine for about 1yr. All of a sudden "BAM" my remote no longer works with my sling 350. I have spent countless hours trying to reset and re configure everything with no luck. I am thinking something changed with slingbox and it no longer supports my MP2000NA. let me know if you have any luck
I am using an arris mp 2000 portal that is connected to my arris gateway system. they portal is working fine with my slingbox 350 but i cant flip channels using the remote on the tv screen. i have tried all the remotes on the list but to no avail. i thought it was the sling box so i tried using it with a motorola 3400 and it worked perfectly.