Sorry, but I don't know why you are not seeing an upgrade button and the other missing menu items in your left menu column when you open your return back up. I think it's better to use a computer instead of a phone since menus and websites sometimes behave weirdly on a phone. But you've said you already tried a computer and experience the same thing.
The only product that does not have an Upgrade link is the TurboTax Free File Program, which is different from regular Free Edition and has no upgrades since it is fully-featured. You already confirmed above that you are using regular Free Edition, so I don't know why your return doesn't have one.
I'll post in a back channel and see if I can attract anyone else to comment in this thread today.
You could try phoning TurboTax Customer Support, but without PLUS, you may not be able to speak to anyone since Free Edition without PLUS doesn't normally have phone support, so you may hit a roadblock.
Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Monday-Friday.
FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
If you can't get through to Support, you could contact a TurboTax Support agent directly by messaging at Facebook or Twitter. Same hours as above.
https://twitter.com/TeamTurboTax
or
https://www.facebook.com/turbotax
Another alternative is to get a free prior-year return transcript from the IRS website. If you qualify to open a free IRS account there, you can download one. If you don't qualify to open the account, you can still order a transcript to come by mail, but that takes 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript
Or if you wish to get additional ideas and don't get any comments in this thread, you can start a new thread of your own and title it something like: "Need to upgrade to PLUS after filing 2018, but after opening the return with "Add a state", there is no upgrade link in the left menu column."
If you decide to do that, you can use the blue button "ask a question" at this link:
https://ttlc.intuit.com/community/after-you-file/discussion/02/203
