How to complain about service?

I am so upset with the customer service at this company I don't know where to begin.  Back in August 2021 I called At&T to find a landline for a friend of mine in Pine Hill, NJ.  She would lose her phone service when her cable, Comcast, lost power and she wanted a landline.  AT&T (Your customer service representative, Lamar on 8/30/21) said they were no longer installing them.  Therefore we ordered a cingular flip phone and were told we would get a senior discount. I called every month about it and received discounts for a few months when I complained about it until I was told that they were only giving me a discount to get me off the phone.  I finally canceled the service on May 24, 2022 and now I received a bill including a late charge of $7.00.  Your representative today said that I should have been told that the service dates ran from the 10th of the month and I would have to pay the full month.  I was NOT told that at all!!  Now they told me that this was the only way to 'complain"  Is this correct?  I can't find a single address.  I am so disappointed with the service from AT&T!