Flint as HomeRouter (Exit node) and SlateAX as TravelRouter using Tailscale

Please help me set up a HomeRouter - TravelRouter set up.
I’ve bought a Flint to use as HomeRouter and a Slate AX as TravelRouter.
In the home network I’m behind the Spanish Movistar router for fiber.
First I tried the direct approach, I don’t have a static IP.
I tried port forwarding the 51820 port on my ISP router and it seemed that the Flint couldn’t use the NAT, so it wasn’t working, I also tried to use glddns, and the ddns test never worked.
After that I put the Flint on a DMZ behind the Movistar router and still the same, the glddns test never worked.
I cannot put the Flint before the Movistar router, as it is connected directly to the fiber.

So afterwards I decided to use Tailscale. I had to modify the code for the tailscale config in order to the flint to advertise its routes, so I could activate it as an exit node (Brume 2 as a Tailscale exit node - #7 by asksimple)
I did that an it was activated as an exit node. I tested with my mobile phone and it worked.

Now I’m trying to use my Slate AX as Homerouter and using the Flint as exit node.
I can see the Flint exit node and select it in Custom exit node.

However it doesn’t work. After I do that I cannot access the internet.
Afterwards I modify the Luci config in the flint according to the first comment here:

After I did this I cannot access anymore the Flint remotely, but I can still use it as an exit node with my mobile phone or other devices, but not with the Slate AX.

How can I make the Slate AX use the Flint as exit node?

The Flint works as an exit node with Android phones or Windows PCs using the Tailscale App, it just doesn’t work when I try to use my Slate AX to use it as Custom Exit Node.