Have now done following (had to use mobile hotspot to download updates)
1. Factory reset
2. Update to September 2017 security update
3. Update to 1 December 2017 security update (WW_V5.3 19_20171219
Seems to be working fine now.
Any update on this? My daughter's ZenPad10 is doing exactly the same. Reboots after about 5-6 if on home router (tplink ac1600). Absolutely fine when using my phone as hotspot. All other devices in home have no issue with router.