The brick walls can’t be helping. Are they dry internal walls, or might they be external and possibly damp due to precipitation? If damp, that will make things significantly worse.
Can you log the RSSI (on the local emonCMS), so that we can see the strength of the signal being received?
Also, is it possible to arrange a ground plane? This is the easiest way to improve signal strength. There was a big discussion a little while ago - Improving RF signal strength emonTH emonTX
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You can have the ground plane at either or both ends of the link, though the emonPi is seeing a massive signal from Node 8, so a ground plane for the emonPi might actually create problems for that emonTx (something to be aware of).
Is it possible to get the antenna the other side of the first wall? It’s likely that an extension cable through the wall will be less lossy than the radio path (@Bill.Thomson is likely to know the numbers off the top of his head).
Or, do you have a decent WiFi signal where Node 16 is? If so, you could connect the serial (programmer) port to (say) an PiZeroW and send the data straight into your LAN. Or if not there, near enough to have the PiZeroW where there’s a good WiFi signal and close enough to the emonTx for the serial link to work satisfactorily.
Unfortunately, the Pi has only one serial port, and that’s in use already, so you can’t connect directly. But at only 3 m and if there’s a reasonably direct cable route, a serial connection between the emonTx and a serial to USB adapter plugged into the emonPi might also be feasible (especially if you use a lower baud rate).