I have an old WiFi router, will that be a problem for connecting /staying connected to a Mill WiFi heater?

Not necessarily.

If you have an older WiFi router connecting / staying connected shouldn’t be a problem as long as the WiFi router;

• has enough capacity for the number of active devices using it
• distributes b/g signal

Read more about necessary WiFi settings under article: Questions about the WiFi router > WiFi router settings required when using Mill WiFi heaters