Changed plan but still got billed for previous plan for 8 months

It is a long and complicated story.

I upgraded my plan in March. It was 600 Mbps for $60 and a rental modem was included without extra fee for two years. After I placed the order, I got a new rental modem and the speed was as fast as what Comcast promised. Everything looked fine.

However, I still got billed for my previous plan, which was 75 Mbps for $49.95 and extra $14 rental modem fee. The total amount was $63.95. I did not notice it at beginning because the speed was as promised and the fee difference looked like tax.

From last month, the price increased to $69.95 plus $14 rental modem and the speed downgraded to 30 Mbps! I started noticed that I payed extra fees.

I've been calling and web chatting with the customer service for whole week and got very little progress for my account. What I got were:

1. $42 refund on Dec 4.

2. Real issue spotted on Dec 9.

3. Recovered to promised speed, 600 Mbps, on Dec 10.

4. Removal of my own modem from rental modem list on Dec 10.

Unitl now, the plan name and billing amount are still not changed. The rental modem is not found in the system. Even I could not shop for new plan. Everything on my account is super weird.

How to make my account normal without keeping bothering the customer service every day?