I'm going bonkers! I'll pay you 50 dollars to fix me!

I have a Note 4 Sm N910L from Korea. I live here in Japan at the moment. I bought the phone properly over Amazon and its new. I bought a data sim for it and it was working perfectly for about 7 weeks then one morning when I woke up there was no signal I have tried everything - updated from 444 to 501 then downgraded again UP and DOwn Maaany times. If I go near the Docomo store I can get connected again and it will stay connected for the 3km back to my home and then go off again when I reach home and not go back on - so I know the hardware still works ( I guess). It now just continually says Limited service or "your card is not authorised to use this network". Ive tried many other sims in the phone, none work.

If you can give me a step by step COMPLETE guide with links to any files I need to fix it, I'll pop 50 dollars in a paypal account the same day I can make my first skype call again!!! First person's info that actually does the job gets the money.

Love my phone and Im absolutely &%$'ed without it.

Hope you can help (the wife is out buying some rope and a straight-jacket as I type)