Just to clarify and avoid any confusion ref. the VMG1312 model you mention.
I believe the reference is to the VMG1312-B10D model not the VMG1312-B10A.
Yes, the latest FW revision for the VMG1312-B10D, V.5.13(AAXA.7) 15/12/17 does disable Telnet/Busybox access, to the best of my knowledge this has not been fixed/changed by ZyXEL.
The previous FW revision, V.5.13(AAXA.5) 20/07/17 permits full Telnet/Busybox access, as such works with DSLStats.
As far as I know, there are no such issues with the VMG1312-B10A with it's latest FW revision.
Can't comment on the VMG3925-B10B, don't have one, I'am sure your post will be of interest to many other forum members who run this model.