I've got a friend out at Kai Iwi near Whanganui whose VDSL is ~23/10 with occasional hiccups. They've got a Netcomm modem of some sort, supplied by Slingshot, and POTS.
I've found their master jack, so should be able to put a master filter there and use a spare pair on one of the internal wires to run the DSL signal to the jackpoint they actually want - I'll install an RJ11 jack there.
I dimly remember that some modems were better than others back when I was on rural VDSL in 2018 - the HG659 came recommended back then. Now our options are the Netcomm, or what's in my junk pile (some fritzboxes and grey Vodafone homehubs), or to find another HG659. Is the netcomm likely to be good enough or should I chase down something better, and if so, what?
Another option to throw into the mix is to get them to switch plans/ISPs to someone who provides a good modem, and VoIP rather than POTS.