Fixing RAID5 array after replacing a disk on QNAP TS-453A

I have a QNAP TS-453A NAS with 4 6TB disks configured as a RAID5 storage pool. Currently with firmware

Yesterday I discovered that one of the disks (disk 3) had failed, and that the storage pool was in a "Degraded" state.

I quickly got a new disk. The old disk was a WD Red 6TB WD60EFRX, the new one is a WD Red 6TB  WD60AFEX. I hot-swapped the disks. According to the documentation, the new disk should be detected automatically, and the storage pool should automatically start rebuilding ("Rebuilding" state). But nothing happened.