How can I use Office 2011 for Macs still?

Upgrading the amount of RAM in your Mac shouldn’t have changed a thing with Office 2011.

Simplest thing to try first is to reinstall Office 2011. Make sure to apply all available updates.

Cost wise, that’s what you’d definitely want to do first. Buying a perpetual license copy of Office 2016 now will shortly be a waste of money. Office 2019 is due for release later this year. MS has no upgrade paths. You’d pay full price for Office 2016, then again for Office 2019.

(1) You are going to have to bite the bullet and move to Office 2016 or Office 365 or

(2) try an alternative platform such as NeoOffice or Open Office or something similar or iWorks (Pages, Numbers, Keynote) for Mac

https://www.unlockboot.com/microsoft-office-alternative-mac/

But IMHO, if you are working with others who are on Windows platform sharing their files with them and they are using Office the see (1) because you will never hear the end of it.