IIRC that big drop is what happened to everyone with ECI modems on a Huawei cabinet. Originally G.INP was on both downstream and upstream, as tommy, BatBoy and I have been discussing. Then OR withdrew it completely for a short time while they semi-sorted the ECI mess.
They then reintroduced it on downstream only and some say the ECI also received a firmware upgrade sent out online. Maybe yours didn't get that and your line may have been banded. It's quite a fog-bound chain of events! (That's being polite). Or maybe the reintroduction was after you swapped over.
No you can't see a thing with the locked HG612. I haven't got the link for the unlocked software version I'm using at the moment, and Tommy's is the one before. I expect someone else will pop up with it soon - I have a very busy day tomorrow as not done my tax return yet!
Possibly the version in yours is already non-G.INP. The fact that the ECI screwed your line up doesn't men to say that a G.INP HG612 wouldn't fix it, but it's almost certain in my mind that a non-G.INP HG612 wouldn't.
So your problem might not be being unable to turn it off, but just a need to update to use it.
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
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Connection - AAISP Home::1 80/20. Sync 59504/15641kbps @ 600m. -
BQM