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For whatever reason, it looks like the DLM has applied a banded profile capping the downstream speed at 74Mb. So nothing you can do will immediately restore the speed, but if you remove the problem, the DLM might eventually remove the banding. The difference between modem and router is their place in the ‘internet-chain’. Modems connect to the internet directly, translating code and messages, but cannot create a local connection. Meanwhile, routers can read the new messages to create a local network for your devices but cannot connect directly with an internet provider. An important item to note is that for the PPPoE connection a "Service Name" should not be entered. If you have this field in the configuration of the router or computer that you're using then make sure that it is left empty. Interesting that you have a HH5A as that is a Lantiq chipset like the 130 but you don't experience any issues with it.
Vigor 130 with talktalk router? How to set up Vigor 130 with talktalk router?
Provides Annex A modem code 567517 & 562301, and Annex B modem code 567617 & 567212. Recommended for Germany. (G.Vectoring) 3.9.8.4-08v
DrayTek Modem Types
I tried changing the SNR value (-50) like someone had mentioned on Ubiquiti forums but made no difference, it always reports the value at around 8 (unless I'm reading the adsl status report wrong). I had the same problem, identical down to the 73999 sync speed, except, the cab I'm connected to is a Huawei. Notice the heading, "ATU-R Info (hw: annex A, f/w: annex A/B/C)", is explicitly stating "ATU-R" i.e. the local modem/router. Under that main heading there are two column headings, "Near End" and "Far End". Logically, something has to be incorrect about what is displayed.
Vigor130 VDSL2/ADSL2/2+ Modem Quick Start Guide - DrayTek
Vigor 130 modems provide BT Superfast Fibre and ADSL2+ capability and feature one LAN port. This is the standard modem design.
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DrayTek's VDSL modems provide reliable DSL connectivity as well as excellent performance and compatibility, designed to comply with Openreach MCT requirements. NFEC This parameter reports the actual Reed-Solomon codeword size in bytes used in the latency path in which the bearer channel is transported.