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I have pihole (DHCP enabled) in my raspberry Pi. In my Macbook, I have IPV6 disabled and I use VPN as and when needed. When I use the router's default DHCP, VPN connection stays until I disconnect. When using pihole's dhcp, VPN disconnects every 5 minutes and reconnects. With tcpdump, I see a correlation between Macbook's DHCPRequest to Pihole's DHCP server and VPN connection drop. In Pihole DHCP server, my lease time configuration is 168 hours.
Update: It is not actually VPN issue. Wifi drops and connects every five minutes in my Macbook. This happens only when I use Pihole DHCP. I wanted to rule out any IPV6 related configurations. I have disabled IPV6 in both raspberry pi and pihole DHCP and didn't change the issue behaviour.
Bucking_Horn
Before receiving your comment, I have tried another approach and it solved my issue.
Prior setup: Cox modem/router combo -> raspberry pi with pihole with dhcp
Current setup: Cox modem/router combo in bridge mode -> another router -> raspberry pi with pihole with dhcp
With current set up, I don't see any wifi drop. Only downside is download speed has reduced. I'm not planning to revert my set up atleast a month. Once I get into a similar issue later, i'll repost it.
The download speed on your Cox account is controlled by the provisioning of the modem and should not be impacted by your LAN setup, unless one of the routers is bandwidth limited.