Why not use another app? You have so many options with the firestick and side loading
Sounds like provider has only set 1 account up!
Did you try pausing or powering off the box to see if the other one works better? How far are your two devices? Is your wifi strong enough?
Hopefully your IPTV service provider is genuinely supporting 4 connections - contact him first to check
Your firestick, are they on different floors? Could be an wifi interference...
Most smart tvs don't have the memory to cope if there is lots of data mine was same and use to have lots of customers on smartiptv and was always saying it goes off or down soon as they change device or use the website player I have it seems to be lot better
Did you give him the right Mac address for the second one.
You sure you can use your account on more than 1 device?