I am using a V2860ac with Sky Fibre and Sky Q.

The Sky Q boxes are connected over wireless. Every morning the Sky Q boxes claim to no longer have a network connection, despite my other non-Sky products (e.g. iPhone) being connected to WiFi and able to get online. I have to reboot the DrayTek to resolve the issue with the Sky Q boxes.

When I first had this issue, I tried reconfiguring both Sky Q boxes to use Ethernet over powerline adapters and disabled the wireless on both boxes via the engineer menu. However even on Ethernet both boxes had problems, resolved by rebooting the router.

The router is running the latest firmware (3.8.9.1). Is there any reason why this keeps happening?

Can't say if it's the same with you but I had problems with various devices, both wireless and wired, losing connection for no apparent reason. I had input Google's DNS into the router and was forcing it to use that DNS as set in LAN 1. Removing those settings and allowing the router to pick up my ISP's DNS seemed to cure all those odd problems.

Can you hard wire the main Q box into the router temporarily to see if that helps ?

Try binding the Mac's to IP, in case the problem lies when the router goes to re-new the IP. I know the reboot contradicts it, but it may help.