The “baseband” is actually the wireless modem for mobile radio. The modem firmware aka baseband firmware is the software responsible for the cell activities within the entire Samsung firmware. Its tasks include harmonizing antenna reception to cell, enabling communication with other devices such as Blue-Tooth, Wi-Fi and GPS.
Unfortunately there is also official baseband firmware available which is not suitable for custom Rom’s.
An example from my LG G3 d855: One of the last official StockROM Android 6.x v30B (13-00106) = doesn’t work with custom Rom.
Official StockROM Android 5.0 v21C (13.2-0000002) = works and is recommended for android 5.x and higher custom ROMs).
Another experience on my part with a Galaxy S5 SM-G900F see posting #7 Post Baseband updates?
What is the baseband code of the baseband firmware used on your SM-A520F?