Sounds to me like you have a faulty battery heater, or perhaps there's a coolant leak as I think the battery won't heat if there's not enough coolant in the system. First stop is probably to call the dealer and see what they say.

Remember that, so long as you've had the approved services at Hyundai dealer, the Hyundai Roadside Assistance includes Homestart so if you really can't charge they should be able to get you to the dealer:

What’s included with Hyundai Roadside Assistance?

Coverage – UK and Channel Islands
Roadside Assistance – For breakdowns over 0.25 miles away from home, we will aim to fix your car at the roadside
Roadside Recovery – If we are unable to repair your car, it will be taken to a local Hyundai dealer or a local destination of your choice
Home Start* – For breakdowns at home or near home, we will aim to fix your car or take it to a local Hyundai dealer
Onward Travel*† – We will provide a replacement car for up to 2 days or overnight accommodation or cover incurred public transport costs

 

Thanks Duncan - I will try that!

 

We got home from a weekend away on Monday with 20 miles range (planned it that way). Turned on the house heating and went out to the pub to eat. Got home later in freezing conditions with 8 miles range. Apart from the dash telling me the obvious, that I needed to charge I had no issues. Car plugged in overnight for my 4 cheap hours and everything is fine.
As others have said you seem to have an issue requiring the dealer. Needs sorting before the Christmas shut down at the dealers.

 

What kW charger are you using? I'm not sure how the Kona works but I know my Ioniq activates the battery heater at 3.5kW or something. It's never happened to me but I wonder.if using the 2kW granny and a sub zero battery won't work? it's very bad for the battery to charge below 0c apparently.
I might find out tonight when I try and charge in minus 4C.

 

This was also what I wondered. OP doesn't mention what type of charger he is using and I wonder if the battery heater doesn't work from a granny charger.