Bell fibre strange thing
Bell fibre strange thing
In short, your fiber connection has changed over time. Previously it offered 500 Mbps upload and 1 Gbps download, now both sides are 1 Gbps. Your speed test shows you get full speed at the modem but reduced speeds at the switch—500 Mbps down and 1 Gbps upload. Since you switched to direct connection before the change, it suggests the issue might be with the switch’s downstream performance rather than the cables themselves. It’s possible the upload wires are okay while the download wires are experiencing degradation. Since you’re comfortable with your current speed, you’re right to question what’s happening.
You might attempt to run a fresh temporary wire from the modem to the switch. If you regain the original speed, we can confirm it's the old wiring. It's conceivable one or more of the eight internal wires were compromised, possibly due to improper handling near the connector. You could use a basic tool to test the cable for this. Alternatively, if the issue is confirmed, you can reattach the wire properly.
I believe the tool you have for securing cable ends might include a small tester, which could be useful. If the cable is faulty, I think it's likely because only part of a Cat 5 cable is in use, so you could easily replace the damaged section with another unused one.
On gigabit Ethernet, every wire is active; removing any one causes the system to switch to 100Mb instead of full gigabit performance.