If anyone's trying to order one of these Frient heat alarms, a smart home supplier called Vesternet out of the UK seems to ship to the USA. I placed an order through German based smart home supplier, Reichelt, but they cancelled the order since the heat alarm has batteries inside and is deemed hazardous to ship.
Both companies are listed on the Frient site as distributors. I'll update this post if I run into issues with Vesternet.
You can also get the Frient heat and smoke alarms from Amazon UK delivered to the US.
Finally ordered the Frient heat alarm from Europe, that UK based place called Vesternet mentioned above. Device paired fine, has been reporting temps regularly in our garage, and has been online for about two weeks now.
Slowly I'll start integrating it into the Hubitat safety monitor, but for now I have just a notification set if the device detects an issue, mostly due to not trusting it yet to not give me false alarms.
I have yet to test it, but will get around to and report back if I find any issues. It just reports battery, temperature, and "smoke". I assume smoke gets tripped when the heat alarm detects a rapid temperature increase, or a temp over 122°F/50°C as it lists it's temperature range at 0°C- +50°C. It vaguely lists it as "heat based fire sensing". Kinda wish it stated actual numbers for that, but I'll test it myself and make a judgement call.
I think it triggers at 135F but I think it's also a rate of rise device. So maybe anywhere on the range of the change is greater than 15 degrees per minute. Stick it in a box and use a hair dryer.
I think i figured out how to pair it. Had also problems pairing it to my C-7 hub (FW: 2.4.0.151).
You have to hold the button down long time (i think at least 10 seconds) until it blinks third time. I believe it resets the sensor and puts it to pairing mode. Then initiate Zigbee pairing from the Hub side. It connected in seconds. Otherwise always timed out before.
Just for info i purchased the frient SMSZB-120 and it also would not pair with Hubitat, tried all the reboot reset etc on c-7. It would find it but then fail to add it and tell me to reset it and try again. Annoying thing is, i got the make and model because i already have one paired so thought it was guaranteed to work. Clearly not.
Tried it with Home Assistant and it paired immediately! I can also make the siren go off from home assistant and i cant with the one i have paired to hubitat, useful as i can add it into my intrusion alarm.
FWIW it looks like that still hasn't been fixed, then. 
The SMSZB-120 smoke alarm I opened the thread with is distinct from the HESZB-120 heat alarm that's also been discussed. AFAICS all reports of successful pairing are for the latter.
As of late April @mike.maxwell was waiting for a firmware update for the smoke alarm - I guess that either hasn't happened, or you received older stock with the less compatible firmware? 
Download the Hubitat app
