@MR.S 

1. connect controller pc with cable, don't use wifi

This is the way I have it set up.
2. set fixed ip on pc with controller

I set a fixed ip every time. 
3 have you preconfigured the LAN ip on the controller, make sure it is the same ip network as the controller, otherwise you will lose the connection when adopting.

Checked each time for same network. 
4. adopt and enjoy :-)

In the end it seems I have got it working. What I did was: 

• Uninstalled Omada software controller from Windows PC, along with associated files. (This enabled me to adopt again)
• Reset ER706W
• Installed Omada software controller again
• Adopted ER706W - success!
• Hard reset and power cycle of both EAP670, restarted Omada software controller.
• Adopted both EAP670 - success!

The EAP670 showed as "Managed by other" naturally. I first tried hard reset and power cycle of only one EAP670 as the other is in the next building, this didn't work. I tried restarting Omada, still didn't work. Only when both EAP670 were reset and power cycled it worked. Could be coincidental, but I will add it in case it can help someone else.  

I now want to change the IP from 192.168.0 to something else like 192.168.10, along with some other things, but I'm almost afraid to touch anything now that it's working 

  @stole 

good you got it working :-) well change the ip.. i would change the ip let the router provision and when you lose connection with the router i would stoping controller change the ip on the controller and start controller again. You must restart the access points or wait until they get a new IP from the new IP network.