Maybe someone can clarify further, but I'm not sure if YieldStreet's offerings are available to other IRA custodians (to communicate/collaborate with YieldStreet on the asset purchase & later selling) unless you find good one, because it's a custom offering and not generic alternative assets.
I do know YieldStreet itself offers its own IRA program for you to invest in their funds. From when I asked YieldStreet about it, their response it seems was that you could supposedly use your own IRA custodian to work with the YieldStreet IRA for transferring funds and making purchase, etc. but you'd end up paying fees at your own custodian + YieldStreet IRA. The YieldStreet IRA has their own IRA fee. And that YieldStreet fee, depending on your alternative asset performance, doesn't seem like it is worthwhile investment.
If you can't invest in YieldStreets offerings outside of their IRA program, it's not worthwhile investment and better to invest with non-retirement money.