You need to have earned income from a business, so I don't think the dividends could be used for a solo 401K (unless there's some sneaky-Pete way to say that you have an investment business, and this is the income. I'm fairly sure the IRS doesn't allow that). But, on the face of it, it would seem that you could establish a business to manage your rental properties and that this would be earned income for the purposes of the Solo 401K.
The business does not need to be an LLC or incorporated to have a Solo 401K, a simple sole proprietorship will work fine (though there could be good liability-limitation reasons to run your rentals as an LLC, etc). I'm not sure how your business's legal structure might impact the depreciation deductions and other unique aspects of owning RE. You'd have to pay self employment tax (though a small silver liningis that it might bump up your SS monthly checks a tiny amount, depending on what your earnings history has been).
With a Solo 401K, the first $17,500 ($23K if age 50 or older) could be tax deferred, plus about 1/4th of the amounts earned above that (until reaching the cap).
Another plus for a SOlo 401K vs an IRA: If the Solo 401K plan document allows, you can start withdrawing the money in the year you reach 55, rather than waiting to 59 1/2, if you terminate employment in that year. It can allow you to use up more of that tax-deferred money a bit earlier and lower the possible hit from RMDs+ SS at age 70. While this could also be accomplished by transfers to a Roth IRA, this other method is a bit simpler. More
here.