without activation or a subscription, your current office installation will continue to read and view the files just fine, you just won't be able to edit and re-save them or create new ones.
there are a number of 'free' or open source alternative to several of the microsoft office applications (word, excel, and ppt), such as onlyoffice, libreoffice, softmaker, etc. set the default save format back to microsoft office format (docx, xlsx, pptx) for a more seamless transition. if your online drive is mounted in your os, any of these would be able to read/write to it like any other installed application.
older versions of microsoft office (2010 and earlier) may be 'out of date' and unsupported, but they still work and can be bought second-hand for cheap.
there is also free-to-use online versions of microsoft office and google docs (their respective online account required--and their anti-privacy policies apply). these would by default use their respective online storage.
if you are in university, you may be able to get a low-cost or even free microsoft office key or subscription from your school. check with your student i.t. help desk or school-run campus bookstore.
if you work for a larger company or institution that uses volume licenses of microsoft software, they may have a 'workplace discount' for a microsoft 365 sub, it's about $20-30 off per year (the more reasonable 'home use program' does not exist anymore).