How do I suspend my plan while keeping my number?

Hi. I will be leaving the US for around 6 months, and during that time I would like to suspend my phone plan. Currently, my phone plan is prepaid ($40 Simply Prepaid 10GB LTE – Unlimited Talk, Text, Data). I know that I can suspend my account twice a year for a maximum period of 90 days for each suspension and that I will be charged $10 per billing cycle. I still have questions though.

1. what does it mean "billing period"? Is it $10 per month? Or does that mean $10 per suspension period (so around $3.33/moth)?

2. how do I actually suspend it? The instruction on the website says,

1. Go to My T-Mobile and log in to your account.
2. Click PHONE.
3. Click either Report lost or stolen or Temporary suspension, depending on the type of suspension you need.
4. Follow the on-screen steps to suspend or restore your line.

However, when I clicked the My T-Mobile​link, I couldn't see PHONE.

Here is why I see:

and when I clicked "Edit plan," I see this:

The instruction is a bit confusing. Not sure if it has anything to do with my "Prepaid" method??

What IF I just stop refilling $40 a month during that 6 months period? Can I still keep my phone number back after 6 months when I get back to the US??

Thank you!