See the plan you are setting up, and its first invoice, on the billing page
The problem You accepted Founders, stopped before paying, and the billing page said Solo with no invoices — as if the plan and the $29 it starts with never existed.
The billing page now names the plan you are setting up, lists its first invoice as pending once it has been raised, and gives you one link to pay it and start the plan. Solo’s header button is “Choose a plan” and offers both paid plans instead of Startup alone, and a Founders checkout says Founders all the way through.