The physical interface on the router itself only has 1 SIM slot on it. The cell card is not hot swappable, but you theoretically could have another card on-site with a different sim. Power the router off, swap out the cards and you should be good to go.
Your only other option to have automatic failover is to use 2x 881G's and use a routing protocol (or HSRP or something) to fail automatically between the two routers.
