Thanks to tthoben for sending me the details by PM.
I hope he doesn't mind me reposting it here, I'm sure it'll help lots of people!
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Steps to setup Alexa Bridge (RM Plugin)
1) set static IP address for each of your device (i.e. echo/dot, RM Pro, android box) in your router settings
2) purchase and install the paid version thru the Google Play Store, not the lite version which expires in 7 days and become unusable
3) ensure that your android box does not go to sleep (disable all screensavers, power saving modes)
4) ensure that the app is running (tapping back may exit the app, tap home instead to get back to home screen)
5) tick the options "enable Alexa Bridge", "Autostart on Boot", "Keep Screen On for HTTP/Alexa Bridge"
If the above are done and discovery still fails, then likely the solution is to reconfigure your Alexa devices on the android box itself:
a) select "Alexa Device List"
b) delete all devices that were imported (swipe left to delete)
c) select "+ADD ALEXA DEVICE" button below and manually create each Alexa Device Name, On and Off Commands (note that the commands are essentially the codes that you imported from the Broadlink app)
d) while doing that, you will be able to test the On and Off Commands before you save
e) if testing here works, congrats it will work eventually
f) once all the Alexa Devices are configured, ask Alexa to discover devices
g) if step (f) fails try using web interface ( alexa.amazon.com ) under SMART HOME > devices > discover
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I have to say, even before I start working through these steps, I'm not hopeful of success.
My network setup is pretty standard, but I do use US Amazon account with my Echo in the UK, so I'm unsure if this influences anything on the firmware/compatibly side of things.
The problem I'm getting is that RM Plugin is detecting a request from Alexa, but fails on the final step of getting device info.
I've tried it using a different router and even with a different Android device but it all falls at the last hurdle.
Having Googled the error message I'm getting, it seems to be an uncommon issue.
I'm going to give it another go tonight!
RDT