A self-directed solo 401k may be invested in cryptocurrency provided the solo 401k provider offers a solo 401k plan that allows for investing in alternative investments such as cryptocurrency.
A solo 401k from a self-directed solo 401k provider like My Solo 401k Financial allows for investing in alternative investments including cryptocurrency, and many of our solo 401k and IRA LLC clients have invested in cryptocurrency since 2014 when the IRS released IRS Notice 2014-21 which essentially confirms cryptocurrency is deemed property not a collectible.
Video Slides: How to Open a Gemini Account for your Self-Directed Solo 401k