Again, check your link rate. Don't assume that a quality cable will systematically ensure a 10 Gbps link. Asuswrt' network map will show you the link rate of all connected Ethernet ports.
Not every Ookla test server will be able to provide 10 Gbps of throughput. In fact I suspect that very few do (if any at all). Test with other methods, such as downloading from multiple sources in parallel to see the aggregated throughput. And double check that negotiated link rate. If the ONT lacks any UI to check, some ONT will show a different LED colour based on the link rate (my Nokia ONT does). Otherwise, either a plugged router or TB NIC should report it within the OS.Testing wired speeds directly out of the ONT with a Thunderbolt 3 to 10GBaseT adapter was a confusing experience as sometimes I'd get the 2.5Gbps speeds while other times I'll get speeds above 6-7Gbps.