Dual WAN issue - no Internet

[jan]

I have difficulties getting a setup with 2 WAN Gateways on OPNsense 19.7.6 to work properly. My intention was to implement a failover/loadbalance setup.

The problems began after adding the new Gateway (DSLGW) in addition to the existing Gateway (WANGW). Before that, everything worked fine for years

The current situation is:

• When both Gateways are enabled, and the new DSLGW is the active default gateway, my clients can connect to the internet.
• When I disable DSLGW, the old WANGW becomes active, but connection to the internet is lost.
  
• I also have NAT rules set up to proxy https traffic from the internet to a servers in my DMZ. This traffic is coming in via the WANGW.
  This now only works if the new DSLGW is inactive
  

So I now have a setup where either the inbound (NATted) traffic via WANGW OR the outbound internet traffic via DSLGW passes through my firewall, but not both.

Now I am now a bit lost as to where to start troubleshooting:

• I cannot see blocked packets in the Logs
• I already tried to work through the documentation on Gateways an MultiWAN
• I cannot find details on some settings like "reply-to"
• I tested tons of small changes to settings all with no positive effect
• I tried to reduce the setup as much as possible (no gateway monitoring)

Any hints please?

Thanks
Jan