Your monthly funds arrive at the start of each coverage month, so the first money lands on your coverage start date. If coverage begins January 1, that's when you can start spending on eligible health expenses. Those expenses have to happen during your coverage period — anything dated before your start date isn't eligible, even if the card would go through.
Premiums are different. If you brought your own plan, you can order your card and add it to your insurance company's portal for autopay before coverage begins — that's exactly what it's for. Through open enrollment, your card is set up to cover your January premium during the fall, so that first payment lands on time and coverage starts cleanly on January 1.
If your card isn't ready when you need to enroll, pay with a personal method, add your plan details to Thatch, then submit that premium for reimbursement.