Whilst the webUI has a page for updating modem firmware, it never works and never shows an update, the reason for this is that modem firmware upgrades are sometimes tricky and take a long time, which is the excuse provided.

Anyway, if you want to you can upgrade your modem firmware fairly easily.

First SSH into your router. Now just update packages (just in case) with

opkg update

Now install the modem updater package with

opkg install modem_updater
opkg install sshfs

Now enumerare your modems with

modem_updater -g

And you'll receive a list of modem's, their current firmware, and available firmware, for example:

Modem List:
[0] Quectel EG06-E | USB_ID: 3-1 | Firmware: EG06ELAR04A20M4G | builtin:true
Available versions:
EG06ELAR04A04M4G
EG06ELAR04A06M4G
EG06ELAR04A07M4G
EG06ELAR04A08M4G
EG06ELAR04A08M4G_BETA1208
EG06ELAR04A20M4G
[1] Quectel EG06-E | USB_ID: 1-1.2 | Firmware: EG06ELAR04A20M4G | builtin:true
Available versions:
EG06ELAR04A04M4G
EG06ELAR04A06M4G
EG06ELAR04A07M4G
EG06ELAR04A08M4G
EG06ELAR04A08M4G_BETA1208
EG06ELAR04A20M4G

Now make sure you switch to the modem you are NOT going to upgrade, so if upgrading the second modem, switch to the first and visa-versa, OR, plug your router into a wired internet connection and set that as the default route, you cannot upgrade a modem that is being used for internet access at the same time.

To Update use the command

modem_updater -i <ID> -v <VERSION>

For example, to upgrade the second modem to the latest firmware use

modem_updater -i 1-1.2 -v EG06ELAR04A20M4G

This will take a fair amount of time so don't panic, and maintain a connection to the router over ssh until its complete. Once complete the modem will reboot and that itself may take a couple of minutes so again, don't panic. You do not need to reboot the router.