Well, I got some progress here.
It turns out that I had 2 OSC devices in Reaper working at the same time (my USB device and Touch OSC in my phone). They had different configuration files with different set ups, so they where interfereing with each other.
Once I deleted the one from my phone it works! Here is my setup:
DEVICE_TRACK_COUNT 1
DEVICE_SEND_COUNT 0
DEVICE_RECEIVE_COUNT 0
DEVICE_FX_COUNT 1
DEVICE_FX_PARAM_COUNT 100
DEVICE_FX_INST_PARAM_COUNT 0
DEVICE_MARKER_COUNT 0
DEVICE_REGION_COUNT 0
REAPER_TRACK_FOLLOWS REAPER
DEVICE_TRACK_FOLLOWS LAST_TOUCHED
DEVICE_FX_FOLLOWS FOCUSED
And also:
FX_PARAM_VALUE n/fxparam/@/value
With this setup the controller works with the focused plugin window, which is what I wanted
.
Also, If I select a different track in Reaper by clicking on it (let's say track 1) the controller now works with the plugin track 1. Then is funny, but if I click on the plugin window belonging to other track (let's say track 2), the controller keeps working on track 1, until I select track 2. Then I can select again the plugin window that I want and the controller keeps focused on it. I said thst is a funny behaviour because it looks like a mix in between track select/focused plugin style. I like it, btw.
Now, I also noticed that when clicking on the plugin window Reaper sends back a lot of parameters. I guess these messages are precisely the ones I need to update the controller. So at the end it looks that Reaper did it automatically... which is cool.
Now I'm trying to figure out which message/value corresponds to which parameter. Till now with OSCii-bot I only have managed to see the param numbers (not names), and also I can't see the values.
Cheers!
Alvaro