It is sad I cannot mirror link s21 ultra. And also my s9 plus. I'm very disappointed samsung. This is my last phone unless there is some improvement for people like us who hugely relied on mirror link.
MirrorLink is Betamax to Google's VHS. I personally found that, although Mirrorlink was reliable, it was just not as feature-rich as Android Auto. Maybe if Samsung devoted a bit more time and energy into it, MirrorLink could well have been a decent rival to AA.
What car have you got? There is a good chance that there is a firmware update for its head unit that will then let you pair with AA. I was always able to use either (until the end of MirrorLink) in my 68-plate Seat Ibiza, but I found a website that offered an upgrade for my head unit. That update improved my Bluetooth connection, plus a few other refinements. Maybe its something for you to consider?
Speak to your dealership first, but be mindful of them quoting you a ridiculous price for updating your head unit. I was quoted +£200 to update mine, but I found a motor parts company that sold me the software download for £20!
As a side note: if your head unit also has built-in satnav, you can also get a map update from: www.satnavstore.com. the mapping software for my car cost around the £35-40 mark.