RB850Gx2 connect with RB450

Dear All,

I need help in configurating the following setup. I’ll try to be as clear as possible.

Setup:
RB411 with LTE and Wifi
RB850Gx2 with VPN tunnel.
Routers are connected in this way: RB411 Ether1 —> RB850Gx2 Ether1
Ether 2 on RB850Gx2 is configured as a WAN port with dhcp client
Ether3, 4 and 5 are bridge for lan purposes.

When a client on the lan sides connects via cable they need to get internet acces either via WAN(Ether2 RB850) or LTE (LTE0 RB411) or Wifi(WLAN0 on RB411). The wifi in this case is configured as a client.
So far I have no problems setting this up. Just a matter of setting route distances.

The following options need to be implemented and that is where my problems are.
How to:
Get the WALN on the RB411 used in the lan bridge on the RB850?
Set up a IPSEC VPN tunnel to route all traffic trough which connects either via WAN, WLAN(as client) of LTE?
Manage all the settings from one routerboard if possible.

I have tried many things including vlans but i just keep losing track of everything.
Buying other hardware is not an options because the RB850 needs to setup the VPN IPSEC tunnel because of the hardware acceleration.

If somebody could help me it would be greatly appreciated.

Greetings