Yes, the routing kit cable (with the magnet on it) is stuck inside the chain-stay. I've tried every direction with the magnet and it won't move. I have tried pulling on it pretty hard but not full force because I don't wanna accidentally burrow the cable deeper if it's caught on something. Unsure how it could get that stuck, I was guiding it using the magnet and then suddenly, boom, stuck and I couldn't back out.
Just to clarify, the routing tool + the rear derailleur cable housing is stuck inside the drive side chainstay? Is your bottom bracket installed? If it is, take it off and see if you can wiggle the cable out of the chain stay from the BB opening. As a last resort, you can use a metal clothes hanger to see if you can fish it out. Just be careful though, you don't want the clothes hanger stuck in there as well! In fact I keep a clothes hanger wire and a paper clip as one of my frame building tools for situations like this!

Saying that, the G056/RF25 was actually my easiest bike build, I had very little issues building this frame out.
