I was going to recommend RevampedFMRadio but I see your device isn't either Qualcomm or MediaTek.

There's some discussions on XDA and Reddit I found where someone wanted it on an S10/S10e and the responses were that the Exynos chip does not have any FM radio functionality at all.

I looked up your Exynos 850 chipset and Samsung's datasheet says it does have an FM radio, as you yourself surmised by it being in the stock ROM.

There is this app (seemingly without anything prebuilt) for a different Exynos chip, but given it hasn't been updated in 3 years and the chip difference, I wouldn't have high hopes.

Otherwise you might have more luck asking the LineageOS maintainer why FM radio functionality is missing and if they could add it.

Do Samsung phones even have FM antennas? As far as I know they have to be a minimum length and old school feature phones had a hacky workaround where you had to plug in earbuds to listen to the radio because it was using the earbud cable as an antenna. Modern Samsung phones don't even have headphone jacks so I can't imagine they support FM at all. Probably your best bet is to use an internet radio service that has access to the stations you like.

https://f-droid.org/packages/org.y20k.transistor/

although not in FM...

This might be an option, although I am not familiar with lineageOS.

Apkmirror link

Get the app Radio Garden. Gives you access to almost every radio station in the world, but uses streaming

RFM Radio popped up when I searched on f-droid (IzzyOnDroid) just there, says it's incompatible with my version of Android and it requires root but it does actually use FM radio.