If your Mac supports it, you will want to upgrade to macOS 10.13 High Sierra, or newer.
How to get old versions of macOS - Apple Support
Have a backup created before the upgrade.
Use Safari to download the upgrade kit.
If you are not sure about upgrades, use 🍎 > About This Mac, and post the Mac model and Mac model year. With both the Mac model and the Mac model year both and we do need both the model and the year, we can look up your upgrade options. If this is the MacBook Pro shown in the footer, which year?
If you cannot upgrade to a newer macOS version, you will will then have to retrofit some certificates.
The following directions—I lack any way to test these—look reasonable for retrofitting a certificate on OS X 10.11 or earlier.
https://mjtsai.com/blog/2021/09/24/some-web-sites-will-stop-working-with-el-capitan-and-older/#comment-3538503