Thanks, I found some instructions on that site regarding the OS I'm using and manually installed ISRG Root X1, which seems to be working for me now.
Feel free to Delete my comments here, so they don't mislead anyone.
That cert is good till 2035, so you should be ok for some time. Might be a good idea to post a link to the instructions you followed here so that others who stumble onto this can do the same.