Thank you for your answer. Below my comments:
I have a main router with 4 Wan ports, one dsl, one cable, and previously using usb an LTE stick. The LTE is used as a backup in case of failure.
Now the LTE stick died, and I decided to get a replacement, so I bought the RuxX09. I have currently one SIM of german telekom inside. German Telekom allows you to select an APN where you get a Public IP Address (not natted), so I am even able to run Services like microsoft exchange over LTE using dynamic DNS.
If the Rutx09 is set to bridge mode I have the Public IP on my main routers WAN port. Works good (and fast). However, since the Rutx09 allows sending SMS using a simple API I thought I use that for a service which sends a second authentication using SMS. But for this I need access to the Rutx09 Web interface.
My first thought was:
Set port 1 on the Rutx09 to Vlan1, connect it to my routers wan
set port 2 on the Rutx09 to VLAN2, give the Rutx09 a new MAC on this Interface, connect this to a VlanTagged port on my internal switch, and assign the rut09 an IP using dhcp. Then, I thought I can access the SMS service of the router from my network. But this does not work (the problems which occur I already wrote in the first post).
I have now accomplished this task differently:
Bridge mode on, and set the Rutx09 to a static IP 192.168.220.1, connected one lan port of Rutx09 to wan3 of my router, and set the Bride mode copy send all traffic to the MAC of wan3 of my router. Connected another lan of Rux09 to wan4 of my router and set the WAN4 ip of the router to 192.168.220.10. Now I can speak to the Webservice of Rutx09. However, at the cost of losing one WAN port - which I might need. If there would be another solution without sacrificing a precious wan, that would be better.
If not, well, then ok, but I am still wondering whats happening on the Rutx09 (on my first try), when creating a layer2 Vlan where no connection should be possible between two ports but the 1st ports gets assinged an IP by of my internal networks DHCP server.
Also, If I insert a second SIM, can I use this normal and still use bridge mode for the 1st SIM?
I hope I explained it now so you able to understand?
It now works with a single wan. I did setup WAN3 for DHCP (receiving the packets from LTE via bridge), and also setup a new virtual wan on the same WAN3 port using the static IP of the Rutx09. So, no WAN port wasted. Kind of solved this problem. Unknown the question if I can use a second sim in non bridged mode, and if Bridge mode is used, if a Layer2 vlan or Tag based Vlan will still work if bridge mode is used.