The code on Schedule C is not a NAICS code. Line B on Schedule C says "Enter code from instructions." The instructions for Line B say "Enter on line B the six-digit code from the Principal Business or Professional Activity Codes chart at the end of these instructions." Choose one of the codes in the chart in the IRS Instructions for Schedule C.
In my mind, cryptocurrency mining is a financial activity, not a computer service. I would use 523900, Other financial investment activities. That's just my opinion. If you don't agree with me, use 518210, Data processing, hosting, & related services.
It's not really critical what code you use. The code doesn't affect your taxes.
