No problem. We are clear now.
This is possible.
Here is a thread which covers this topic.
https://community.ipfire.org/t/2-wan-interface-2-internet-providers/2204
You basically need to create a dialup profile for your main adsl connection and another profile for your mobile 4G connection. Then the 4G profile is set as a backup on your maion adsl profile.
Wiki page about dial up profiles.
https://wiki.ipfire.org/configuration/system/dial
Here is the wiki page on Network Hardware Compatibility - 3G/4G section
https://wiki.ipfire.org/hardware/networking#3g4g-umtslte-modems
Here is the wiki page on setting up the 3G/4G profile.
https://wiki.ipfire.org/installation/red/mobile
If you have any further questions on this or have any difficulties then come back. I have never tried this myself but there are definitely other forum members who have worked on this and will be able to help you.
Good luck.