Yes, you can set up the Orbi router as an access point and connect it to the Salt router,
Or set up the Salt router as a bridge and use the Orbit router as a router. But you would have to clarify this with the support from the provider.

I would highly recommend this: if both WLANs are running (fibre router and Orbi), interference cannot be ruled out. Since the performance of the Orbi is certainly much better, I would cut the fibre WLAN and only use the Orbi for WLAN and routing. I am thrilled with this: over two floors in a brick-built house I have my full speed in the furthest corner, as it comes from my modem (unfortunately only copper...). Enjoy!

I have UPC and have it running completely as a modem only. Orbi takes over router and WLAN. I then get approx. 800 Mbps on my S20 Wifi6. However, this is of no use when UPC goes berserk. I had a mixed router from UPC still on and Orbi, which always gave me problems.