I have a TP-Link MR3420 router. (It's definitely V2 or higher, I don't think it's really relevant except for the flash file)
Currently, I use it on the stock firmware to provide 3G connectivity to my WiFi smart sockets. But now I need to set up some IP cameras via Powerline, because my main router is too far away and I don't want to use a long Ethernet cable and a separate switch.
So my goal is as follows:
Use the 3G stick to provide connection to my WiFi sockets
Use the LAN ports as a "dumb" router to connect the MR3420 to my main router via Powerline and to connect the cameras to the MR3420
Is it achievable with OpenWRT? If so, how do I do it correctly?
AFAIU I need to:
Bridge the 3G and WiFi interfaces
Disable DHCP and set the IP and gateway manually
Am I right? Is there anything else that should be done?
sure... this is really simple. You'll basically just remove the ethernet ports from the lan interface.
Is the wifi smart plug + 3g cellular USB stick stuff working currently?
How many ethernet ports do you need for the IP camera(s)? (obviously one port uplinks to the powerline -- how many downstream IP cam ports?)
Yes, the 3G+sockets stuff is working correctly, but the MR3420, as I've written in the original post, is on the stock firmware right now. I didn't flash to OpenWRT yet because I needed to make sure that my goals are achievable.
I will be connecting 4 IP cameras. The MR3420 has 4 Ethernet ports and a WAN port, but I'll just remap it to an Ethernet port and plug the uplink there.
Ok... so yes, this is totally possible with OpenWrt.
I think I overlooked that bit...
The problem is that your hardware is so old and limited that you will have to run a very old version of OpenWrt that is EOL and unsupported. It will have many known security vulnerabilities and is not considered safe to use anymore.
The latest release version of OpenWrt (technically LEDE) for your device appears to be 17.01.7. That is over 6 years old and has not been supported for the last 4. There has been a huge amount of change with respect to the configuration syntax, and it is likely that people here just won't even remember how to configure this properly (it's like asking people to speak fluent Middle English today -- there are similarities, but a lot of major differences, too).
With that in mind, I would actually recommend keeping your existing configuration as-is, and just buy a cheap unmanaged switch for your cameras -- a 5 port device can be purchased new or used for pretty cheap (~$10 USD).