Thank you for your query.
I would like to double check: RUT240 has 1x LAN port and 1x WAN port. Have you Configured RUT240 WAN port as LAN? Because, if you connected your pc to RUT240 LAN port and Siemens S7-1500 to RUT240 WAN port it wouldn't assign an IP address, since WAN port is used to provide internet connectivity for RUT240 itself. A quick test that you could carry out is, plug Siemens S7-1500 plc to the LAN port of RUT240 and check whether it fixes the issue. However, I would still recommend to configure the WAN port as LAN so you could have the computer connected as well, detailed instructions can be found here:
Legacy firmware version: https://wiki.teltonika-networks.com/view/WAN_as_LAN
RutOS firmware version: https://wiki.teltonika-networks.com/view/Setting_up_WAN_as_LAN
If so, please check if DHCP has been enabled on RUT240 LAN interface.
1) In order to do so, please open the routers WEBUI and navigate to Network -> Interfaces -> LAN -> edit(click on the pencil on the right hand side).
2) Scroll down to DHCP Server, make sure it is enabled. In addition, you can assign a static lease using Siemens PLC Mac address so it would obtain the same ip address every time.
Note: You have indicated, that you are unable to change the IP address of Siemens PLC device remotely, if you remember the IP address that has been assigned previously, you could set a static lease with Siemens PLC Mac address in order to assign this IP address to the PLC device. In addition, a good practice would be disable and again enable the DHCP Server on RUT950 LAN interface, because you are unable to reset the lease remotely from what I understood.
3) Regarding lldp and dcp. None of these by default are support on Teltonika Network devices. However, Since our RutOS software is based on Linux OpenWRT, you can download and install the package manually:
After successful installation, type in lldpd --help to view all the available options. lldp is an open standard protocol, however, dcp isn't. It is a Cisco proprietary protocol, therefore, in order to enable dcp you would have to do it via lldpd. Type in the following command: lldpd -c
Note: If you are currently using a legacy firmware version, I would recommend upgrading to RUT2_R_00.07.01.2
Download link here: https://wiki.teltonika-networks.com/view/RUT240_Firmware_Downloads