Need advice on really long fishless cycle. The problem - lack of nitrates

Hello Thor,
Thank you very much for the quick response.

#1 After adding NH4Cl I measure NH3 concentration In 30-60 minutes, it's around 1-2 ppm
#2 I'm using API freshwater master kit for NH3, No2 and NO3 measurement. I'm aware that NO3 measurement is tricky - first need to add 10 drops of chemical #1, then shake _vigourously_ for at least 30 seconds, then add 10 drops of chemical #2, then shake _vigourously_ for at least 60 seconds, finally wait 5 minutes. With the same kit I was able to measure NO3 20-40 ppm some time ago, so I'm confident that step #2 is done the right way.

Let me comment on the NO3 20-40 ppm measurement. 4 weeks ago I measured NO3 20-40 ppm but at the same time I had really high NO2 - around 5-10 ppm.
Also my tank stuck at high NO2 (> 5 ppm NO2) for a week or two. In order to finish cycling I did the following #1 increased temp from 78 to 86 degrees, #2  did 50% water change #3 reduce NO2 concentration to 1-2 ppm