Hi there Jay here,
I currently have this setup on my home network: ISP issued gateway (modem/wifi router) that has two LAN clients 1) an IPTV DVR receiver (i have cable subscription). 2) Connected to Netgear MK62 mesh router WAN port. Becuase of the unique nature of IPTV service, the gateway from ISP cannot be removed, however, Ive enabled IP passthrough which gives the MK62 the Public IPv4 and Ipv6 address, and I have like 40 Lan clients behind the mesh router. Now I have the Mango router with exroot configured and installed Adguard Home with the intent to serve DNS requests to my LAN clients. Looking through the forums and also asking on netgear forum, it appears that as long as the DNS device (Mango router in this case) belongs to MK6https://forum.gl-inet.com/c/routers/52 Lan, it will never work because Netgear would not allow A LAN ip for DNS lookup if that LAN ip is part of the same subnet. I was thinking bridging option on netgear or the VLAN, but that may complicate configuration. However I came up with the idea that it should have it connected to my ISP gateway, so it can be a upstream DNS server , and just type the ip address on netgears DNS settings. Will it work? Do i need to use port fowarding in netgear or at the gateway? Thanks for reading my long post and helping me 
First, using Mango to work as Adguard Home will not work. It is not powerful enough.
Then you want to use Mango as the DNS server, right? If your MK62 does not allow it, then I don’t think it is appropriate to do such a setup.
Well yes I know its not possible. Cause I tried by trial and error and ended up crashing. However if place the mango router at the gateway LAN, my guess it may work. When IP passthrough is enabled (as explained by ISP support) it will pass along the public IPv4 and 6 addresses to whichever device you specify by its MAC address and will forward all external traffic to the that selected device. In this case the router, Netgear, becomes “main router”', checking connection window stats I see that the ISP gateway gives netgear a dns server adress of 192.168.1.254, which happens to be the gateway address where you log in and make changes. So my thought is placing the mango router and giving a static IP i.e. 192.168.1.100, and then tell netgear to use that address. since its not same subnet as netgear LAN
The DNS 192.168.1.254 is a private IP address and would not be assigned by the ISP gateway when it has been configured in passthrough mode. The DNS address is sent by the Netgear MK62 router’s DHCP to the connected PC’s and client devices.
The private IP addresses 192.168.1.254 and 192.168.1.100 are actually on the same subnet 192.168.1.xxx.
As alzhao stated, Mango is not powerful enough to run AdGuardHome and I think there is another forum thread saying that it works, but is extremely slow.