Default Question about flashing a TW ROM

Hey guys, so I've been having issues getting a GPS lock since I flashed an AOSP ROM last week. One recommendation made to me is that I flash back to a TW ROM, get a GPS lock, and retry the AOSP GPE ROM I have come to love (pure vanilla android wins in my book). The options available with TW ROMs vary a lot, so I'm a bit afraid to flash the wrong one. I'm still on MDK firmware, and would prefer to leave it that way as it's about as unlocked as the GS4 gets. Would flashing a TW ROM necessitate that I find one that is explicitly MDK to avoid a firmware upgrade, or will a ROM not touch my firmware version like an OTA would? Sorry if I am asking a dumb question, but I'd appreciate the help!

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