Well u dont look as a newbie to xda u seem to have a lot of knowledge. I did understand ur method but just choose a different way, i.e, i had made a nandroid backup of my stock rom prehead to flashing lineage 17 so what i did now was just restored the modem files from the backup and still nothing seemed to work. Funny fact, i later realized that my sim itself is down cause other operator sims were working fine when i checked. Even i am from india having a bugged up sim of idea .PLEASE NOTE: I am not a professional/developer. In fact I am new to XDA. I just came here to get an upgrade for my phone, and I was not disappointed .Therefore, all I state is based solely on my personal experience.
So, before doing what I stated in my post, I tried to flash just the "modem.bin" file from my stock J500F ROM using Odin. Unfortunately, it did not fix the problem.
By the way, do you have the stock ROM (the original Samsung Touchwiz ROM) for your phone? If yes, you can extract the files you need(including modem.bin) from the stock ROM using 7zip.
Also, be aware that for some reason, you won't be able to flash the modem file using the latest version of Odin. I had to use Odin 3.07 to flash mine and could not flash it using Odin 3.14.1.
Further, I can give the files to you but please make sure your phone is the same model as mine(J500F) before flashing. If not, you will have to find the stock firmware for your model and extract modem from that. However, as I said, flashing just the modem did not work for me (it might just work for you though ).
If you want me to share the files nonetheless, would you like me to e-mail the files to you? Send me a message about it.
Also, if this doesn't work, do what I did. Wipe Dalvik/ART Cache, Data, Internal Storage, Cache, System from recovery, also wipe the modem. Now flash the Stock ROM. Check if Mobile Data works and please note your APN settings if it works. Now, if it works, flash Lineage OS again, and enter your APN settings.
By the way, are you saying that for you, Mobile Data doesn't work EVEN ON THE STOCK ROM?
Well i guess i can help u about audio issue ur facing. Just ensure that audio files ur playing is .mp3 and not .m4a or any else format, if u want to use formats other than .mp3 i would suggest u to download vlc media player(works pretty fine). But if ur facing problem with .mp3 files also then the problem is unique to ur device cause in mine its fine.(mine is also J500F)
Also a load tnx to @Zakaryan2004 due to him my this device out from garbage is usuable for daily purposes now, really very happy.
And dude an advice from my side, sharing of gmails and personel info on any forums is not recommended as for security concerns.