Vlan across three RB

I am not an expert I have gotten to this point from youtube videos and reading the forums. I run a hotspot with three locations, using 4 mikrotik RB. My original net source is giving me headaches and I have gotten a new source that I want to use and load balance with the existing supplier. (yes my supplier knows what I am doing) I am buying bandwidth to resell ( I always see that question asked)
The new source is coming iin from a tower that currently has a AP on it. I want to use the existing link via vlan to get the new supply in the back out over that link. I am not getting it to work.
Attached is a sketch of my current setup which I will describe here also;

b ----a-----c----d A has 3 ports switched with an AP, B and C has 3 ports bridged with 2 AP. D is the new source n is the existing source, X is the router I want to use to load balance
| A,B &C has IP .7.0 address with A running a hotspot and PPPOE server
n–x d has an .10.0 address connecting to a .10.1 gateway
Can some one help me in creating a VLAN from D toX. or direct me in the right path I have looked but none of the examples work and I have crashed my network twice already.
netmap.png

Can you simplify you map.

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simplified
map.png

Can you arrange remote ,I can help.

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