Cellular Modem not recognized after flashing new firmware

I bought a ZBT-WG3526 cellular router with ATT sim card preloaded with OpenWrt from a company that has gone out of business. I could not renew cellular service, so I updated new firmware hoping to override the current restrictions (only ATT available) and add other carriers (tmobile). Now I cannot see the cellular modem to select carrier (or anything else).

How do I get the driver for the QUECTEL EP06-A 3g/4g cellular sim card?

Is there something else I should do?

Try Modem Setup for GL.iNet GL-E750

A QMI modem would need these packages and their dependencies:
kmod-usb-serial-qualcomm kmod-usb-net-qmi-wwan uqmi luci-proto-qmi
You will need to temporarily connect the router to the Internet by some other method to install packages.
Also install picocom so you can access the modem with AT commands for testing.

Compatibility with different carriers is determined entirely by the modem and its firmware which is separate from the router firmware. The EP06-A is not specifically certified for T-Mobile but unless it has been locked to AT&T (which would be unusual for a device not sold directly from AT&T) it will probably work by installing a T-Mobile SIM. It covers most of T-Mobile's bands except 71.

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I installed all software as instructed. I still don't see where I can manipulate the modem.

Under interface, I was able to add QMI Cellular, and got the following error after reboot:

Error: Unknown error (NETWORK_REGISTRATION_FAILED)

This is not a surprise, because I still have the AT&T sim card installed that is no longer activated, and new T-Mobile card doesn't arrive until Monday.

Is there any way to get a "modem interface" with the ability to select desired cellular bands, and bandlocking capabilities? I had that before re-flashing to the new version.

FYI... I dont know how to use the command lines

There's no real standard for modem control, so such an application would need to be written per modem. Since OpenWrt is intended to run on almost any hardware there aren't such apps.

What is typically done is to get the AT command manual for the modem and manually submit AT commands with a terminal application like picocom. This only needs to be done once as the modem setup for a carrier is should be persistent.

There is a very good chance that the original firmware can work with T-Mobile and it also would of course have GUI integration with the modem.