I have a routed wireless network with sort of X-mas tree topology.
Each client is behind CPE/router that is NAT'ing client's LAN network to CPE IP.
每个AP IP分配的组合ed CPE's and from their connections are routed to default gateway or by routing tables from top to bottom.
X-mas tree has several /24, /25, 26/ etc. networks in its branches and border gateway (top) connects to internet and nat's every client to its unique public IP.

Now I have one client A that somewhere in tree's branch need to have PC's of client D somewhere else on a completely different branch to make part of same network.

CPE of client A can ping CPE of client D directly and vice versa over my internal network. As said, both LAN's are behind nat firewall. So PC on LAN of client A can reach router of client D and vice versa. But each client's PC cannot reach directly other client's Pc's. (Because masquerade nat)

Which tunnel/bridge is now recommended to connect both networks like they are one and the same physical network?
Should I use Vlan? But did this not have repercussion on wireless?
Should I use PPtP, EoIP, GRE, IPIP, L2TP or whatever?

I have been reading manuals of each several times but to be honest need an advice on when to use which tunnel.
Actually MT should make a tutorial so ´newbee´ in this field can make an decision before he proceeds and start building one.

What kinds of traffic will you be seeing over this? With VLan's unless all your gear supports an MTU of 1504 on the wire you will be stuck with a MTU of 1496 under VLan's, EoIP can get around this but it will increase PPS due to packets being fragmented from double IP headers, Same with IPIP, GRE and PPTP can be tricky to setup so if it's simple you need and the network can handle the PPS I'd try EoIP first

If I use EoIP, does each router in the path in-between the both end routers also have to be configured with an EoIP tunnel interface carrying the same ID and an IP address in the same network? (In example they use 10.0.0.xxx/30)

If not, how does remote tunnel end know where to find other end? There is no route to it and all routers the packages passes have no info where to find the other end of the tunnel. So package is just send to the main gateway?

The tunnel in my instance has to pass at least 6 routers and they all have different networks in-between them...