I need help with Global illumination

Not completely true.

You can enable Precomputed Lighting as if you disable it, reflection captures won’t work.

In the video, I use a mixture of precomputed lighting and light propagation volumes.
I would post my settings here but unfortunately, I deleted my project file.

iPlay_Zombies, get settings from here:
https://wiki.unrealengine.com/Light_Propagation_Volumes_GI

and the Cornell Box.

Mix and play around with the settings until you get the effect you desire.