Lied to and disrespected repeatedly about credit. Worst customer service I've experienced in my life.

I signed up for an internet plan for $30/mo for the first year, I had to change the start date with an agent on the phone, and he told me the new rate is $35/mo, but he said he put a $60 credit in my account to make up for the difference. That credit never appeared. Yesterday I spent another hour and a half talking with support to try and get that credit applied. I was lied to multiple times and treated with constant disrespect. I only received a $40 deposit despite a customer service agent promising me he would deposit the full $60. He said I'd see the remaining $20 shortly, but to no surprise, I never did. I've lost hours of my time trying to solve this issue and I cannot stand to have to explain this issue to another customer service agent.