Neck Training/Rehab

Hello, I’m putting this under pain and rehab but with a bit of an asterisk. I’ll explain.

I had c-spine disc replacement in 2013- a prosthetic disc between C5/C6. The bones ended up fusing around it later on anyway because I was young, in my 20’s. So it fixed my problem (the nerves running to my right trap, lat, pec and tricep muscles were completely occluded and I had atrophy, this enabled me to slowly regain balanced strength and not have massive pain all the time). But as a small trade off I lost a little bit of ROM.

Fast forward, a decade later, I still get occasional issues with my neck/upper back. Including lately waking up with my pinky/ring finger on one hand or the other being numb. I do NOT want to have to deal with a blown disc again so I want to try to improve the health of my neck and upper back. I looked into directly strengthening my neck to rehab/ pre-hab as it were. A chiropractor recommended I not do any concentric/eccentric training and just do isometrics.

I looked into the “iron neck” but holy cow that thing is overpriced.

So what do I do? Is actual active full ROM training bad in my case? Part of me feels like it’s not, and I just need to be very conservative with the weights but could use any old neck harness with super light weights. Or is isometric really the way to go here? From reviews everyone who buys the $$$ iron neck says it’s worth the money, nobody has regretted it. I guess my hang up is, is that chiropractor correct that isometric is the only way for me to safely train my neck given the fused c5/c6?