How to configure mwan3 or iptables to use specific interface for OPENVPN

I have a working openvpn server configuration on an omnia with two external interfaces (wan & Lte).
On this omnia I use mwan3 for load balancing (failover mainly) where wan is the normally used external interface and Lte is the failover interface.
When I connect to omnia openvpn server from the Lte interface, omnia responds from the wan interface, the default mwan3 interface.

I need the openvpn server to respond from the same incoming interface (Lte).
I tried setting the following rule (wan2 is the Lte interface) in mwan3 but it is not working:

Active ipv4 user rules:
0     0 - wan2_only  all  --  *      *            15X.XX.XX.XX          
1002 72472 - wan_wan2  all  --  *      *              
Active ipv6 user rules:
2262  337K - wan_wan2  all      *      *       ::/0                 ::/0

I am wondering if anyone can help setting mwan3 for this senario.
Alternatively, I could probably use iptables but I have no experience.
Googling I came up with the following serverfault page, but I need someone experienced tell me if it applies to my case.
Finally, there is this openvpn policy based routing package which I am wondering if it is safe to install and use on omnia.

Hi lampra. I use this package and it works absolutely fine on the Omnia for my needs!