I live in the flatlands and 95% of my climbing training is done on the treadmill. It’s obviously not the same as Fell training, but it’s consistent, hard work, measurable, and nowhere to hide (the physical action of actually having to slow the treadmill down on a hard interval is a good psychological barrier to keep me working).
Still manage to get 40 miles/10000ft per week average, when combined with club sessions, but I won’t pretend it isn’t mentally hard sometimes.
As you’d probably expect from such a regime, I find I can “climb with the best of them” (so to speak) but I do struggle on the most technical ground. But since I applied a lot more commitment to training my results have improved significantly, and the fact I use a treadmill has not really held me back as of yet...