Alliant credit cards are lo longer earning rewards when making payments through Plastiq. The Alliant Cashback Visa Signature card was a good option for these payments as it earned 3% cashback the first year and 2.5% after that. You can still use Alliant credit cards for Plastiq payments, but you just do not earn rewards. These payments will not code as cash advances.
You can see a chart of what works for Plastiq here and this page here.