Could you elaborate more on the solution you seek? And what does it have to do with using UDP? UDP is communications protocol, that is fully supported by RUTX08 so if yours devices will communicate in using it, there is no problem at all.
Please provide more information on yours topology, because right now RUTX08 are portrayed to have two IPs, LAN and WAN, in same subnet, and I assume you made a mistake drawing topology since Mobile Unit IP is shown as 192.168.2.101.
Either way you cannot have RUTX08 with WAN and LAN IPs on same subnet.
If you want mobile units to have static IP of 192.168.10.101 and be interchangeable between locations and one another (assuming you are using only one Mobile unit per location) bellow is network configuration that should work, but please correct me if I misunderstood yours solution:
Siemens PLC IP: 192.168.11.2
RUTX08 nr1 WAN IP*: 192.168.11.91
RUTX08 nr2 WAN IP*: 192.168.11.92
*Assuming that RUTX08 connected to Siemens PLC via WAN port
NAT (masquerading) on RUTX08 nr1 & nr2 is ON
RUTX08 nr1 & nr2 LAN IP: 192.168.10.100
Mobile unit IP: 192.168.10.101
In this kind of of setup would let Mobile units send data to Siemens PLC.
If you want Siemens plc to be able to send data/connect to Mobile units you will need to set up Port forwarding.