RT-AC88U & 4G-AC53U // dual WAN // VPN

[archie_n]

Summary:
I'd like to access my LAN/the resources in my LAN via a VPN-connection with a mobile broadband connection.

Network properties:
RT-AC88U router with local IP 10.100.200.1 connected to a high speed network with private WAN IP 192.168.1.x and capability for dual WAN,
4G-AC53U mobile broadband router with dynamic public WAN IP. Router IP: 10.100.150.1

Question:
How can the network be set up to achieve that:
- all devices use the high speed network for "ordinary" traffic
- the mobile broadband connection is active (connected, not standby) and can be used for OpenVPN-connections

Additional info:
- I've tried:

- using the 4G-AC53U as "primary" router with the high speed network as primary WAN and the mobile as secondary and the RT-AC88U handling the LAN . However this results in DDNS not functioning.
- using the dual WAN functionality of the RT-AC88U (primary WAN = high speed, secondary WAN = 4G-AC53U) together with load balance in order to keep both WAN-connections active and direct traffic to the high speed port. Unforunatly this resultet in eiter the LAN-resources not being accessible via VPN or LAN resources using the mobile broadband of the 4G-AC53U instead of the high speed land.
​

Challenges:
- dual WAN configuration: the secondary WAN is standby only, unless "Load balance" is activated (which - among others - implies that AiProtection cannot be used)
- in a RT-AC88U dual WAN configuration with the 4G-AC53U supplying the secondary WAN, I can establish a VPN-connection with the mobile broadband router, but neither with the RT-AC88U VPN nor with RT-AC88U-LAN

Any ideas in order to get thing sorted?