Welcome to the community!
I would like to clarify that the Wi-Fi only crashes when specific device is connected which is the iMAC. Other device(s) connected to the Wi-Fi does not experience wireless dropout not unless the iMAC(s) are connected. Feel free to correct me if I am wrong.
There is a possibility that there is a problem with the compatibility of your iMAC Wireless (Wi-Fi) Frequency affecting the Archer VR1600's Wireless network to drop or to get disconnected.
We did an article that you may find helpful about Wireless Network available here.
Wireless Channel on your modem - what is it and why should I change it?
Easy Steps to Improve Wi-Fi Connection In Your Home
Let us know how it will go.
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