Hello @Hank21, 

That is the way I thought should work but did not. That is exactly what I did, my controller is setup with the site to site vpn and the remote ER7206 is setup as standalone with an entry site to site vpn, the tunnel is up, but when the controller adopt the ER7206 the tunnel goes down do to the controller reconfigure the router overwriting the site to site conf that is in the router. 

BTW I was able to do it this way, I hope this help another person that get this problem. I did a factory reset in the remote router, set it up as stand alone but did not configured the vpn tunnel. I port forwarded the ports 29810 to 29814 in the main office pointing to the controller and in the remote ER7206 configured the controller URL discovery. This way the controller was able to discover and adopt the remote router thru internet without a VPN site to site,  then when the router was adopted, I configured the router and the site to site vpn tunnel in the controller ( configured in both routers/ one in each site). 

Thank you.