Is your Mac being managed by a school or business that may be restricting access to newer versions of macOS? Are you checking from an admin user account and have you unlocked the Software Update pane of System Preferences?
You can find links to various macOS installers within this Apple article which also includes optional instructions for creating a bootable macOS USB installer (I highly recommend you create one just to be safe so you have more options if something goes wrong with the upgrade):
How to create a bootable installer for macOS - Apple Support
Users have reported that Firefox may have problems accessing the link to the App Store, so use Safari or a Chromium based browser such as Vivaldi or Google Chrome.
Does your laptop still use the original Apple SSD? If not, then you will not be able to install Monterey since the Monterey installer will refuse to install a system firmware update. Reinstall the original Apple SSD and upgrade to Monterey, after which you can reinstall the third party SSD so that you can upgrade to Monterey while using the internal third party SSD.
In theory you should be able to boot into Internet Recovery Mode using Command + Option + R to access the online macOS 12.x Monterey installer.
Make sure to have a good backup before you attempt to upgrade macOS in case something goes wrong during the upgrade.
You should always have frequent and regular backups.