First of all, I’m not sure if this post should be on routing forum.
As before, excuse my english, I’ll try to explain my case as simple as posible, because my network config is a little bit complex.
One internet connection is managed by a ccr1009, getting public ip via dhcp on a vlan from the isp. firewall and nat are enabled. ont is on ether1, and a part of my lan is also in ether1, joined by some ubiquiti antennas. I thing it’s not a problem because vlan is like an idependient interface. so everything is working.
In another place I have another gateway that is not in use, waiting for your answers 
, just a cablerouter getting public ip, nat and firewall.
on ccr ports 1-4 are switched, ether1 is master and the others are slaves.
on rb750gl all port are bridged.
before answer my questions, please, keep in mind that if I have a ccr, it should manage the hardest work and traffic, the 750gl sould route traffic only in case of failover.
another thing to be considered is that I can’t change any part of the cabling
1st question: what’s the best way to get load balancing over both internet connections? remember, one gateway is the ccr and the other is a cablerouter that host the public ip and obviously has a lan ip.
2nd question: what’s the best way to get failover in case of one ubiquiti link goes down? it means both parts of the lan and routers can’t communicate each other, so I need 2 dhcp servers running and routing the traffic of each segment of the lan
3rd question: how can the ccr select an appropiate gateway to send traffic based on number of hops or lattency to that host. I don¡t have any idea if BGP can solve this part and, of course, I don’t have any idea to configure it.
I have another question: my ftth internet connection is from Orange Spain, that uses a livebox router that provides the internet connecion and VoIP. Of course I’m not using it, but I’d like to get the telephone working. I was thinking about how to get telephone working. When powered, livebox connects to vlan832 to get the public ip via dhcp and then it connects to sip server to estabilish the voip session. router can be configured via tr69. If I have the ccr getting the public ip though the vlan, how can I configure another port to connect the livebox and make the telephone working? I supose I should configure another vlan with same id linked to ether6, i.e. and with a dhcp server assign a private ip, orange dns servers, needed to resolve sip server name…