That's pretty much what I figured but it's only because I've used an "unlocked" firmware that I'm even able to see the maximum achievable speed on the router.
I assume you mean the Openreach modem then, not your router.
My broadband basic info/help site - www.robertos.me.uk My domains,website and mail hosting -Tsohost. Internet connection - Plusnet Value Fibre.
"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I found it quite easy to do - I tried and failed with it disconnected from the master socket, but I succeeded with it connected to the master. It was quite easy, as long as you have the unlocked firmware saved on your PC.
Make sure you have the Huawei EchoLife HG612 and not the alternative ECI modem.
You obviously have to follow the instructions to the letter, but if you're not confident, why risk it?