Thank you for all your suggestions - for now, v166 is connected and appears to be stable. I managed to find some time this evening to convince the tribe to use my phone's hotspot while I test a few things.
There was a question on if this is my first 3rd party router - its not, but the first in a few years.
Here is how I got on:
-I'm on a Huawei cabinet ~150m away. Just wanted to validate, as I know this could be a factor.
-Before disconnecting the HG612 3B (unlocked with Howlingwolf's firmware B030SP08), I checked the xDSL status page (attached). I'm not qualified to tell if this is all as it should be. My observation is the lower than expected downstream sync compared to attainable.
Swiftly moving on to the Draytek, connected it directly to the laptop and opened the webUI
-Firmware on the Vigor 166 is the 4.1.1 UK version
-Attempted to connect directly via PPPoE and then realized I have little time to learn how to do this in Windows 10.
-I changed the modem's LAN IP config so that it can work on my router's subnet with a static IP. I'm hoping this might help enable access the webUI later. Reboot and reconnect via laptop
-Left modem in bridge mode, set DSL mode to VDSL2 only, changed MTU to 1500 (reboot again)
-Connect P1 on Vigor to WAN port on router, and P2 to one of the LAN ports (this is also how I had the HG612 setup)
-Restart router, connect laptop to wifi, direct browser to the modem's LAN IP. Draytek login page loaded - at least this works!
-Check WAN status on router UI: connected. Public internet working.
-Back to the Draytek webUI. Sync rate same as the HG612 (59.9mbps down, 19.9mbps up), errors all 0, down SnR is lower (9dB vs 13.9dB)
-I spent a couple of hours checking the system logs on the router every few minutes, connection is still up.
So at least for now it appears to be working and stable. The next few days will tell more.
EDIT/UPDATE: after several hours of stability the WAN connection started to cycle again.
If you are using Merlin's firmware, here is a definitely working way of accessing modem interface through the router:
1. Give your modem an IP that is outside the router's ip range. (Your IP numbers 192.168.0.1 for modem and 192.168.1.1 for router are ok).
2. Enable jffs partition.
3. You need to add two scripts to the /jffs/scripts directory, namely wan-start and nat-start scripts.
wan-start script :
#!/bin/sh
ifconfig `nvram get wan0_ifname`:0
192.168.0.2 netmask 255.255.255.0
Note : the IP number shown in bold must be one more than your modem's IP. If your modem's ip is 192.168.0.1 then it must be written as 192.168.0.2.
[Moderator edited to add the bbc code markup for bold text at the IP address, so as to correspond with what is written in the above note.]nat-start script :
#!/bin/sh
iptables -t nat -I POSTROUTING -o `nvram get wan0_ifname` -j MASQUERADE
Before adding scripts, you can check if this works for you easily. Just SSH to your router and enter the commands in the scripts (you can use copy-paste). Then open a browser session to your modem (192.168.0.1 in your case) and there is your modem interface.
Adding scripts just makes sure that the commands are run on router startup.