Prepaid plan change to $15 promo plan and grace period before cancellation.

I've been using a $35 prepaid plan ($30 with autopay). Last week, I've changed that into a $15 plan since I'm outside of the States now and not going back until after about 4 months. I still wanted to keep my number so I changed my plan to a cheaper one. However, when I go to my prepaid account page, it says that I need to make a payment to start my service. I've been using an Autopay so I thought they will also charge me a new monthly plan fee automatically but somehow they are not doing it. I guess my new plan is not compatible with the Autopay? Does this mean that my account is not activated? Or does it just mean that the service has stopped?

My real question is, when I go to the account information page, it says that balance is 0, and the balance expiration date is June 18th, 2020. Does this mean that I can keep my number (my account won't be canceled) until that date? and once I make a payment before that date, the expiration date will be extended?

I'm a little confused about this because it says that my $15 plan hasn't been started, and in the prepaid terms, it says that I have to pay the first monthly plan fee before 26 days of activation. I'm not sure I need to pay before 26 days regardless of the expiration date because I've changed my plan and somehow it seems like my plan is not activated?

What I'm curious about is that, since I want to keep my number, I want to know the last day that I need to pay to avoid my account being canceled. Would it be before 26 days of activation (I want to know if my account is already activated or not first, though!), or before the balance expiration date, which is about 60 days after from now (June 18th, 2020)? Please give me some clear answers. Thank you!