Ok, but, still: The goal was “Public IP for LTE backup.” “LTE backup” implies “for when the primary WAN goes down,” does it not? In such a case: The static, public IP “goes away.” That’s true regardless of in which order one connects hardwired ISP’s modem/router, Sonicwall, and 20x.
Btw: This: What is Drop-In Mode? suggests drop-in mode has the Peplink device between the ISP’s modem/router and the firewall. I wonder if we’re not having a terminology problem?