VPN dropping when GL,iNet is the second NAT

Anyone has any ideas why I keep getting OpenVPN and Wireguard clients (I use Torguard) frequently drop internet connection once my GL.iNet Brume router is set in DHCP mode, behind my main home router? This is despite trying to 1) open all ports on the main router to point to GL.iNet IP’s address or 2) putting the GL.iNet in primary router’s DMZ and then confirming all traffic going to the GL.iNet with no problem.

Interestingly the problem completely disappears and the GL.iNet and its VPN function flawlessly with no problem whatsoever once it is set to PPPoE mode with main router passing PPP through to the GL.iNet. I would prefer the GL.iNet to stay in DHCP mode so that I can use the main router’s WAN failover and load balancing facilities rather than just passing the PPPoE through it. Anyone knows where the problem lies? Thanks in advance for any suggestions.

Port may not matters.

Does your main router has a problem with udp?

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Many thanks alzhao for your interest in my question even though it is not directly a GL.iNet issue. YOu are always so helpful. Main router has no issues at all with opening ports or UDP and even putting my Brume within a True DMZ, I still end up with VPN dropping after a while. I am pretty sure it is not a GL.iNet problem as VPN never disconnects when my main router is in bridge mode passing PPPoE directly to GL.iNet. Main router is a Draytek Vigor 2860 if this is useful info. I wonder if any of the networking gurus on here can shed some light. Thank you again.

Like to help but cannot have an idea.

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No worries, you are always most helpful.