Post [Q] How do I ENABLE MODEM to add frequencies on c6603? (Don't mean flashing baseband)

I have a c6603.

Want to "enable the modem"
not just flash new baseband or firmware.
(Tried that already)

Can anyone provide me with info on "enabling" the modem?
I've seen it referenced in threads, but can find nothing on how to do it.

I get H+ at 850 MHz on my c6603, but not H+ at 1900 MHz

The c6602 (which has same modem), is enabled for H+ at 1900MHz

Flashing c6602 firmware and baseand has not worked.

How would I "enable" the modem to expand the frequencies it uses?

Then I could flash c6602 firmware with the baseband that uses H+ @ 1900 MHz,
because the modem would then be "enabled" for the 1900 MHz frequency.

Thank you for any and all help!