Cables can and do fail (I have had several do so) - so first job change to a new Cat6a one (for better shielding).
Next check that the network card drivers are up to date and that it hasnt defaulted down to 100mbps - look in the network settings.
Also to do it properly and rule out software issues can you do it this way...
Connect a 1GB enabled computer/laptop, with up to date drivers, via a NEW and working Cat5e/6a ethernet cable, directly to the Hub which you have put into “modem mode”
This ensures that NO other devices can be connected
Test speeds at https://speedtest.samknows.com/
- try on 2 different browsers.
If they are still low – boot your device into Windows safe+networking mode - to disable any potentially interfering software - and try again.
There are many posts on here (I have a list of ~30!) where QoS software, unknown/flaky software, old network card drivers, corrupted browsers, bad cables or other connected devices are limiting speeds on tests.
Report back what that gets.
I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired, 1 WiFi, 1 on PA) Hub5 in modem mode with Apple Airport Extreme Router +2 Airport Express's. On 250Mbps, Talk Anytime Phone, x2 Mobile SIM only iPhones.