I have an LtAP (ROS Version 7.4) with a Sierra MC7455 installed. I set the Modem driver setting to MBIM, and set the router with at-chat to AT!USBCOMP=1,1,1009. The configuration of ROS is factory default, with the exception of setting the APNs and setting the Modem to MBIM mode.

I have both a T-Mobile SIM and an AT&T SIM in the router.

From the LtAP (via the web UI) I can ping 8.8.8.8, and use the resolve command to resolve, for example, www.google.com.

However, from any device I connect to it, I cannot ping and I cannot resolve domain names.

Do you have proper NAT masquarade set for traffic originating not from the router itself but from LAN?
Router knows what to do as it knows/has the address assigned to modem interface so there is a flow back and forth.
If there is no masquarade then local IPs go over to the WAN side and then are "dissolve"