After the sub-router restarts, can you log into the main router and look for the client sub-router with an IP address assigned by the the main router DHCP server (e.g., 192.168.8.x)?

Are you assigning a static IP address to the sub-router on the same subnet as the main router (e.g., main router - 192.168.8.1, sub-router - 192.168.8.99)?

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