I think you're going to really struggle with this, honestly. I don't have an answer, and I'm by no means an expert, but I can't fathom an easy way at the moment where you could fuel for long rides without eating carbs.

You've had no problems in the gym because your gym workouts are probably short enough that you can get through on stored muscle glycogen. There's no need to refuel because you have enough "in the tank" to get you through and you top it back off with your post-workout and subsequent meals. Endurance exercise isn't the same animal in a lot of ways. Muscle glycogen can carry most people about 90 minutes. After that, or really even before that because once the stored glycogen is gone you "bonk" or, as you describe it, "hit the wall." On longer rides you have to keep fueling. So, you have to refuel during your long rides and your low-carb diet restricts you from eating the things that are easiest, both in terms of readily-accessible energy and easy on your digestive system.

My immediate thought is that, since you won't be able to eat anything that's quick-access energy, you're going to have to plan ahead and start refueling earlier in your rides and probably more frequently. That's because it's going to take more time to digest fats and proteins and convert it to usable energy. As for what to eat, you'll have to find some on-bike foods that match your dietary needs, won't go bad in the heat in your jersey pocket, be easy to eat on the go, and not destroy your digestive system if consumed during moderate to heavy exercise. I'm afraid I'll have to leave it to you as to what options that leaves you with.

Good luck!