Using Telstra 4G backup SIM for FTTP


I've got a TP-Link Archer C7 which does a very fine job of PPPoE with FTTP, and I have no intention of changing. But Telstra have sent me a replacement modem, which includes a backup SIM for when the fibre is down (if ever).

Has anyone got any configuration advice on which (I guess 4G USB modem) works alongside OpenWRT 21.0.2, with the 4G modem as a backup?