Here we have 420V between L1 and Neutral and 220-240V between L1 and L2
That makes no sense.
In a symmetrical three-phase four-wire wye system, the three phase conductors have the same voltage to the system neutral. The voltage between line conductors is V LL = sqrt(3) V LN
The sensor can safely measure the voltages between neutral and any of the lines is a standard three phase system.
Please post a link to an official document describing the system you are talking about.