I think ypu have discovered exactly why you are "expected" to use the Openreach modem.
I was reading a believable article a few weeks ago, saying that the VDSL2 standards, (re firmware capabilities), are a minefield still, compared to ADSLx. Don't forget there are several different VDSL2 setups possible at the DSLAM end - Openreach currently using 8c? Which as I understand it will change when they increase the top speed. To 17something?
So anyone's chosen VDSL2 router may fail to get on well with the DSLAM.
All a very woolly explanation sorry - just what I remember of something sensible.
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