Here is a snapshot of the current environment. I live on 5 acres and need to get wifi to some remote areas like a barn.
Broadband ISP provided Microtik Router ( I disabled the Wifi and use ethernet to my Main Wifi Router
which is a Dlink Archer C9 In AP mode)
From there I have one ethernet Port going to a TP-Link Powerline Adapter
At my farthest point I have a TP-Link EAP225-Outdoor dedicated access point connected to a powerline adapter. This works great.
Lastly, I have an RV on property and use a GL-iNet Slate 750 in wireless repeater mode connected to the TP-Link EAP225-Outdoor.
Soooo. with the new environment I want to use the GL-AX1800 connected to the ISP. I am not an expert on Bridge, Repeater, or access point terminology. However I do want to keep it simple and safe and not introduce issues like double Nat or double DHCP…
Will I be safe doing the following.
Connect WAN port of Flint to existing port on Micotik (ISP) Not sure what mode to use
Connect a port on the Flint to existing Powerline Adapter
Continue to use RV Flint Router in wireless Bridge mode
I do plan to enable the VPN and Adguard features to protect my entire network…
Please be gentle with me, as I said, not an expert but know enough to get myself in trouble… 
If I understand your objectives correctly, you want to replace your Archer C9 (actually TP-Link, not D-Link) with a GL-AX1800 and be able to used VPN and AdGuard.
Since your Archer C9 is in AP mode (not Router mode), your Mikrotik Router handles all NAT and DCHP on your network. You can replace it with the GL-AX1800 in AP mode 1-to-1 in the same way. However, I believe VPN and AdGuard runs in Router mode only.
In this case, it is perfectly fine to run the GL-AX1800 in Router mode even though there would be double NAT and DHCP. This is exactly my configuration with the ISP modem/router having wifi disabled and a LAN port connected to my main Asus router WAN port via Ethernet. I do not have any problems and achieve rated ISP speed (150Mbps down/15Mbps up).
I do not really notice the ISP modem/router is there (subnet 192.168.0.x) and I only log into it every 1 or 2 months to reboot it for good measure. My internal network (subnet 192.168.8.x) is all behind my main router. Also, this give 2 layers of firewalls that may provide greater security.
Please let me know if I have misinterpreted your objectives.
You were spot on!
Thanks for addressing the Access Point… I should have clarified… I did not want to continue that way. I want to be completely independent of the ISP. No ISP no DHCP and my network dies. cannot even use my barn camera.
This new router gives me the option of tethering in the even of an ISP outage. So back to the point of being able to run router mode with VPN and Adguard. From what you say, this should be an easy upgrade… Seems I just need to set up the router with the same SSID as before and swap routers…
I guess we dont care about the double nat and DHCP since we on a .8.x subnet?
Oh on that note… The Slate that I use in the RV (Wirelessly Repeated) is also on that same .8.x
Will I need to change anything on that now?
When your internal network has a different subnet, then there is no problem with double NAT and DHCP. Just set up your Flint GL-AX1800 in Router mode with subnet 192.168.8.x and DHCP range within that subnet.
Your Slate GL-AR750S does not have to be in Router mode, which is just the default with its own NAT and DHCP. You can switch to Extender mode via MORE SETTINGS → Network Mode, so that it will be on the same subnet as the Flint main router.
If you want to keep your Slate in Router mode, then it has to be on a different subnet from the Flint. You can either have the Slate on 192.168.9.x. / Flint on 192.168.8.x, or vice versa with Slate on 192.168.8.x / Flint on 192.168.9.x.
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Awesome! I think I want to keep the Slate as router mode since I use it that way when I travel with the RV.
I use my RV as an office while parked on property and it works fine as it is with the 8.x
I am taking your advice and just change it to a 9.x one I install the Flint.