For the time being, if it's purely for observing/logging connection stats, the Huawei HG612 is the modem to obtain as it is very easily unlocked (software only), stats are easily obtained in a familiar format & scripts are available for unattended logging 24/7 and graphing at will.
If you are looking for an ECI to unlock for research/interest, then I wish you the best of luck as it does appear to be a trickier modem to unlock & obtain reliable stats from.
One does have to question why BT really decided to lock these modems from users actually being able to see how their connections actually perform.
We wouldn't buy cars without speedometers, oil & temperature guages, would we?
Maybe car manufacturers have slightly less inferior (not fit for purpose) equipment to hide from unsuspecting customers though.