All set up for 3Ds Max as we do with items. Please check it Some basic guidelines for preparing 3D models for R.Design

After creating the room we need to do the following steps:

Make sure the top of the interior room floor is higher than 0,0,0 Axis 10cm: it is at 0,0,10 to avoid it emerging with the outer default floor.

Detach room walls to single meshes and name them as Wall_01, Wall_02...For example with a typical room, walls will be detached into 4 meshes and named Wall_01, Wall_02, Wall_03, and Wall_04.
In 3Ds Max convert the room to Editable Polygon \ select polygon mode \ then select each wall face and hit Detach then Enter Wall_01....Do it again for other faces.
When we detach a wall like this we can apply materials for each wall mesh in our app if not the wall material will be applied for the whole room.

Detach Outer_shell:
Like we do with inner walls we do the same with outer walls. When we detach the outer wall we name it Outer_Shell_01, 02...This will help light leak when we use Lumen lighting in Unreal Engine 5.

Detach Floor: Select floor inner, detach it and name Floor. If there is more than 1 floor then select each floor and detach and name Floor_01, 02...

Ceiling and Roof:
Select ceiling inner and detach it and name Ceiling. If there is more than 1 Ceiling part then select each Ceiling and detach and name Ceiling_01, 02...
The Outer part of the Ceiling will be named Roof