First, a solo 401(k) is not a specific or different type of 401 (k) plan, it is basically a marketing term used to describe a streamlined and simplified 401(k) plan document designed for use by owner only 401(k) plans. Such plans are often drafted to accommodate a 401(k) plan covering only business owners (and their spouses).
A solo 401k plan can be sponsored by a Puerto Rico (PR) LLC as long as the owner-only business owner meets the solo 401k eligibility requirements:
1) Self-employment activity: part-time or full-time
2) Without any common-law, full time employees
VISIT HERE to learn more about the solo 401k eligibility requirements.
Since Puerto Rico is a US territory, a plan based in Puerto Rico is not a
foreign plan. This is important because foreign plans are required to file a Form 5500-EZ even if the annual fair market value (FMV) of the plan is under $250,000.
Puerto Rico has its own tax code. Therefore, just like other 401k plans, a solo 401k plan would need to comply with the P.R. Tax Code. For this reason, make sure to work with a qualified attorney that specializes in the P.R. Tax code.