I have not flown the RB5 and I am still new to using PX4, but on a similar platform I recommend checking parameters in QGC and look for EKF2_Aid_Mask. You can toggle to:
5fcf07a0-841e-4751-a38a-affff3b935f5-image.png

Which should disable GPS inputs and allow you to fly indoors. Just remember to set it back to use GPS when you want to fly outdoors. As Modal mentions, take appropriate safety precautions whens flying indoors even with VIO enabled.