Have you tried connecting your pc or laptop directly to the CPE and then testing speeds?
If you are going to do that you would need to copy the Tenda router MAC address and then insert that mac address in the ethernet adptor settings on your pc or laptop to "spoof" the mac address or it won't connect. Then try and do a speedtest connected directly to the CPE. If speeds are still slowish then you ruled out the router as the culprit.
Morning. Can you try with another PC? We’ve seen a few cases where Ethernet drivers can’t hit 1G, even though it’s synced at 1G. On that note, update drivers and confirm. Vumatel require 2-3 tests connected directly to CPE using different MACs (not your router and ideally 2 different computers).