@anarchi It's probably crashed, not turned off and you'll need to power cycle it. Surprised it lasted so long, mine rarely went much more than a full week without locking up, I believe due to having too little RAM.

You should setup the auto-restart function. I had mine restart at 5am every day, weekly wasn't quite enough to reliably stop it crashing.

[EDIT] Just remembered I ended up putting it on a timer switch and physically cutting power each night, can't remember exactly why though. This POS has many,many ways of failing.

@JerryAtrick Thanks for this. That's disappointing, it seemed to work so well while I was at home. You don't seem particularly impressed with its reliability, is that a general thing?

Thanks again.

@anarchi it's reliability is almost non-existent if you don't have perfect reception (and I wouldn't count on that making much difference).

It launched with major bugs (SMS from your carrier can't be viewed,LTE band selection was a mess & usually got it very wrong, carrier aggregation random, needs regular rebooting - a sign of poor resource handling or insufficient hardware, appalling latency&jitter values +lots of less immediately obvious errors). 2+ years on and support has been non-existent, we're all running beta firmware but having to pick which bug we most need fixing because they can't seem to put all fixes into any firmware release.

The hardware itself is poor, it's the worst LTE connection of ANY DEVICE I've tried, so bad my SIP phones stopped being usable while the same SIM in a mobile gave perfect SIP. So bad even external antenna gave higher signal strength but left it just as confused. I've seen my MR600 work better with both antenna disconnected, there doesn't seem to be anything done right on it.

Yes, you could say I wasn't happy with it. I binned it early this year, bought a cheap ASUS router and tethered a phone to it, magically everything just works, range doubled, speeds more than doubled on LTE, latency fell AND IT DOESN'T CRASH!

You're best option would be returning your's, it's never going to get any support that fixes it. TP-Link abandonned it almost as soon as it launched.

@JerryAtrick Well I’m home and the MR600 is working flawlessly again. I’ll be away again next week so I expect that it will go offline the day after.
This is merely conjecture, but could this have anything to do with traffic? When I’m home my phone, laptop, the TV and a number of other things are constantly connected to the internet through the MR600 wifi. While I’m away I unplug everything so it’s only the smoke alarm connecting intermittently. I wonder if the lack of traffic might be the reason for it disconnecting.
Unfortunately I bought the MR600 several months ago but only started using it recently so it’s past its return date. I’ll soldier on with it for a while longer and see if I can sort it out.