Is this letter of complaint written properly? Thanks in advance for helping me.

Dear Sir or Madam,

I am writing to complain about the low quality of the internet signal on my mobile phone. Also, about a huge mistake on the latest bill, in which the cost of my new device has been charged twice.
I recently bought a Samsung phone at one of your central stores located in Gasteiz street. In the beginning everything looked right, but just two days after buying the phone it started to run extremely slow; the internet connection constantly fails, and it makes it impossible to do a phone call using the Whatsapp application. Because of this bad connection, I can not review my social networks as frequently as I would like. This is extremely important to me, as my job is being an influencer. As a dedicated customer, I hope you will work to resolve my problem. If not, it would be my choice to change my current mobile phone account to another company.

In addition, as I have mentioned before, there has been a big mistake in the last bill and I hope you fix this issue and give me a refund as soon as possible (I've attached a copy of the bill).

I hope to hear back from you about these issues. I can be reached by email at [email protected].

Yours sincerely,

MR