Attempting to modify my services and I am unable to checkout from my cart.

I followed all prompts to modify existing services and had everything in my cart ready to checkout. Upon trying to proceed to the final step, I reach a page saying to try again. I have done this numerous times and also tried via the app to no avail. I have also contacted support through both chat and phone and both are claiming the deal I had in my cart is unavailable to them. I am just trying to adjust my services to better fit my families needs and no one at this company ever helps. I am a 10+ year diamond tier member (whatever that is worth). It should not be this hard to modify my plan. Our entire 400+ home neighborhood gets fiber in just a few months and they are going to lose a massive customer base unless there is some value added.