Question about D-Link Dgs-105

This could be due to the network cables used/installed. For 1 Gbit/s, the cable must have 4 pairs (8 wires). For 100 Mbps, only 2 pairs (4 wires) are necessary. There are two possibilities for such a scenario. A wire is broken in a cable. Or it is an older economy installation where only 2 pairs per socket were used.
Try replacing all patch cables, maybe one of them is defective. If this does not help, check the permanently installed installation.

The Ethernet speed change is automatic, each port is 1Gbit/s and if a device is connected that is only capable of 100Mbit/s, one port will automatically deliver 100Mbit/s, but the other four will continue to deliver 1Gbit/s.