Here is how to upgrade to Plus and pay for it: https://ttlc.intuit.com/community/payments/help/how-do-i-pay-for-plus-in-turbotax-online/01/27001#M51
This link explains what returns can be accessed free and what returns required an upgrade to Plus: https://ttlc.intuit.com/questions/3355524-when-does-access-to-my-prior-year-turbotax-online-returns-expire
There is no reason why I should have to pay for my resent tax return that your company did for me, if I knew this was going to be an issue I wouldn't have filed with your company, that is the reason why I declined in 2018.
You wouldn't have to pay if you save your return each year before Oct when online closes. You should always save both the PDF file and .tax files after you file in case you need a copy in the future (like to get a loan) or if you ever need to amend your return.