From what I've read over and over, unless you have a G900p or some other variant (there are two) you're not going to get a working FM radio on an S5. Something about a very small hardware change that makes enabling the FM radio impossible on the other platforms. There is no rom, kernel, etc. that will ever be able to enable the FM radio on those units. This issue has been discussed ad nauseum to the point that the conclusion was that it wasn't going to get unlocked.
However, as an ATT customer, I was determined to get a Galaxy S5 that had an FM radio so I bought a G900P (Sprint) and went through the painstaking process of unlocking and rooting it. One of my favorite phones aside from the screen not working in the cold weather. I can enable LTE to work on ATT but the only snag is that if I lose LTE signal, I'll be downgraded back to 4g and it won't reconnect to LTE unless I specifically tell it to. I use shortcut master (lite) to access these functions. There were some other issues that required tweaking but in the end the phone is 95% of what I wanted.
wrong. every S5 variant (there are about 9 actually listed here Versions: G900F (Europe), G900I (Asia, Australia), G900M (Vodafone), G900A (AT&T), G900T (T-Mobile), G900W8 (Canada), G900K/G900L/G900S (Korea) found here
https://www.gsmarena.com/samsung_galaxy_s5-6033.php) has a fm chip in it, just many mfgs have chosen not to have that app functionality included in their firmware as it requires a bit in the kernel for it to work from the mfg. AT&T isnt too known for being super customer friendly, at least in that era, and custom roms that utilize touchwiz are based on stock kernels from the carrier/mfg, if the kernel didnt allow for the fm radio to work stock, the likelihood of a dev figuring it out is VERY low
oops totally disregarded the second part of your post. heres the thing about the g900p, its built to run on sprints cdma 3g network for voice and texting as well as low speed data and their own bands of LTE. Now, at&t uses completely different bands of LTE than sprint in most areas and the sprint G900P can be unlocked to be able to run on many domestic networks, but not all and the FCC doesnt force sprint to make them THAT cross compatible. Firstly, LTE is just a marketing term for 4G that was coined in the midst of customers not knowing what the 2g to 3g thing even really did, so marketers decided that LTE (long term evolution) should be used as its different enough that they thought they could fool cdma users that they were on to something completely new even tho it was just good ol GSM 4g data. And in that time, there was a huge battle over which was superior, cdma or gsm. CDMA lost that battle in a hilariously huge way and the beginnings of that was the concession of cdma carriers using gsm for their data. SOOOOOOOOO, carriers who knew that there was now a cross compatibility between their networks now where it was physically impossible before were keen on hampering their phones from being used as much as possible but still complying with the FCC (cell phone carriers are dirty a$$holes like that). So, the sprint a$$holes decided to make a firmware for their proprietary radios allow them to work on at&ts older HSPA+ network, but not their newer 4G network. To be honest, im shocked you got LTE to show up at all. Matter of fact, i seriously doubt that it was actual LTE at all, but your phone showing that symbol erroneously which is 10,000 times more likely as the bands at&t uses arent active in G900P models from the factory. There is a thread on activating all the bands, but its a mind numbing PITA and requires you to be on a very specific factory ROM/Kernel. Short story, good luck with that. Just sell it and go buy a more current phone that also has a FM radio in it from one of the hundreds of MFGs that sell GSM only phones that will work for you, give you 4G faster than HSPA+, are cheaper and far more functional than an aging S5