Hi Pappiger. I can understand your confusion due to the very similar type designation.
- The ASUS RT-AX86U Pro does not have a slot for a SIM card.
- The ASUS 4G-AC86U, on the other hand, has the option you are looking for. So there is a slot for the SIM card.
Here is the LINK. Have fun and good luck!

ASUS 4G-AC86U

No, the router does not have a SIM card. Ideally, it needs wired access to the Internet.

No, it's not a 5G router.

Why would a router need a SIM card?

No