The scene falls off

• when you import 3rd party models they get converted into something Dimension can use... that takes Ram
• when you open a Dn file [everything 3D] Ram takes the load
• assuming your system still has Ram the lighting starts to be built... this takes even more Ram and mostly Cpu because Adobe doesn't use your Gpu to build this rendor

if you share the Dn file we can take a look at what steps will improve your final image... without the Dn to work from you are pretty much, on your own mate so good luck

Can you share a file for testing. Did you use only assets from Dn or import them from other apps?