Fairphone is still maintaining Android 11 for the FP3(+) due to the Fingerprin situation for now, but updates need to be installed manually (no reset needed, but you need to download and sideload it). What Android 11 build are you currently running? Is the popup still appearing despite installing the latest A11 FPOS update?
For more Information also see the following threads:
And also see this article regarding manual installation (subsection ‘Fairphone 3(+) > OTA sideload’)