Have been using 3700v4 for 3 years without any problem (no excuse to buy a new unit!). BB is HGC 100Mb VDSL2. The device is extremely stable. Never encountered any 斷線 issue. Having tried 3rd party firmwares like DDWRT, openwrt and Gargoyle, I would say the most stable ones are Gargoyle and the original stock firmware.
In addition, as far as I know, 500Mb is the limit for this ancient unit. 1000Mb users better go for other options.