Maybe you need to convert the materials to something that Unreal understands, like standard material, or vray. Also not all maps are supported, especially those from 3rd party plugins. I suggest to make some export test with a few objects to see what works. By the way what materials do you use in your 3dmax scene?
here is a link that may help you :How Datasmith Translates 3ds Max Content for Unreal Engine | Unreal Engine 5.4 Documentation | Epic Developer Community