Guardian beams do not render when playing on Minecraft Realms, but they do render in singleplayer worlds.
This isn't an issue with only me, all players of my Realm have the same issue, regardless of their OS, computert specs, resource pack or mods,
Environment
OS: Windows 10 Home 20H2 (19042.1110)
CPU: Intel Core i5-9400
Graphics card: Intel UHD 630
Resource pack or mods: None
Does MC-165595 describe your issue?