It sounds suspiciously like interference between 2.4GHz WiFi and Zigbee. If you haven’t come across that before the bottom line is that Zigbee and 2.4GHz WiFi channels have a considerable overlap. Your hub will have a fixed Zigbee channel (I believe selected from a range when first used) so you are stuck with that but you can force the WiFi to use a channel that keeps as far out of its way as it can rather than using an automatic selection which might choose the worst one (and may vary when you reboot your router).
Ideally you want to minimise 2.4GHz WiFi use, especially with boosters, and get some distance between the hub and the router. Ten feet is often suggested but any distance is better than nothing. Using mains powered Zigbee devices to provide repeating to strengthen the Zigbee mesh can help too.
There are several discussions on this subject on this community and many more on the net to give you more info.
It might not be that at all, but enough boxes have been ticked for it to be considered.