Could you please follow the following steps to upgrade modem firmware to the latest.
When upgrading, router must have internet access via e.g Wired WAN.
Go to CLI: Services>CLI
Password: same as you set to login the web ui
curl -s http://188.8.131.52/download/upgrade_ec25.sh --output /usr/sbin/upgrade_ec25.sh
chmod 755 /usr/sbin/upgrade_ec25.sh
For latest firmware version you can refer to the attached image you can find it in private msg.
Based on modem model you will need to change last line accordingly to fw version to which you like to upgrade. You can see router's modem model in web ui of device Status>Device>Firmware version. Please consider 2 alphabets after EC25 to match with your modem's model . You could also consider date to know the latest modem fw for your modem.
E.g. /usr/sbin/upgrade_ec25.sh EC25ECGAR06A07M1G
Hope it helps.