I was at a festival recently with the same issue. Rural location, good signal strength, but mostly no chance of any data transfer and calls wouldn't connect. We managed ok without, but the only solution that would have worked there would have to have been satellite based as the local cell towers were no doubt completely overwhelmed by thousands of users all trying to get connected at the same time. No mobile based dongle would have been of use there.

I tend to be a bit wary of these paid unlock services - they are often somebody who downloaded an unlock generator from the internet and is charging $20 a time just to type some numbers in. On that vein I went looking for the unlock generator and I found this: ERC Calculator - トヨタ純正ナビ ロック解除コード計算機トヨタ・レクサス純正カーナビのERCコードからロック解除コードを無料で計算。機種別（NHDT・NHDN・NHZT・NHZN・NSZT・NSZN）の操作手順付き。 which seems to do a number of stock radios - I think mine is a NHZN. Source code is also available: https://github.com/cyrus07424/ercCalculator Not tried it as my unlock code has already been set to 0000. Perhaps someone can give it a go?

I have an English translation in Word format Here is the complete English translation of the user.docxof the pdf user's manual. Hope it's helpful.

Many thanks everyone

Looks like Amayama has new ones for £90:

Have you tried an OBD scan? If not I would invest in a JOBD scanner which gives you a good chance of finding out what is wrong. There are threads in this forum (other fora are available) dealing with JOBD scanners. For the really keen there is the Techstream software option available. Personally I would get some Reddex and put that in the tank and go for a good drive to see if that helps.