TBH anything is possibly. I simply don't have enough evidence to know for sure where the problem arises.
The logical way to approach is to change one variable at a time, monitor, then move on to the next combination
For example, I'd like to be able to try different SIMs for a sustained period, but I'm only at this location some days, and the sims are needed in personal phones. Since the problem seems to start a while after restart it's tricky to know for sure.
The usage we made of tethering was lighter than the use as a router - though only perhaps 50% more, but even then the sims are setup differently.
Since I'm on a 1 month contract dumping three isn't a huge issue, though the alternatives are more £. The more interesting Q is whether it's a B535 issue. If it is, I'd have to make a warranty claim.
The biggest issue with all these devices is they are so locked down (plus we have no insight into the network) it's hard to get all the detailed info needed to really know what's going on