I'll rephrase what @frollic has referred to:
If your router comes with OpenWrt preinstalled [...] navigate to System -> Backup/Flash firmware,
then see if you have an option to save the mtdblock(s).
If the vendor has removed this option from their firmware, see if you can do the same from the console. One of several related discussions is here.
The modem calibration is stored on the modem itself, you don't need to backup it.