Need Help configuring fortigate 90d with alot of load balancing/failover

[Xcage]

First of all hi to everyone and any help is appreciated.

The whole thing is a WAN load balancing/failover problem or not actually a problem but a configuration issues.

So i have 2 fortigate 90D's , lets call them regular and backup one, since that what they supposed to do in the first place.

[style="background-color: #ff0000;"]Main[/style] one has 6 different ISP lines connected to it , since each line can provide only 40/3mb of bandwidth and the place needs much more than that and geographic location of the company doesnt have100 mb dsl lines yet.so there are 4 40/3 VDSL lines which are high quality ones, and we almost never have problems with those. main need of the company is stable upload, so we use those for that.

other 2 lines are 500/10 cable lines, which aren't DSL and lower quality isp etc, but provide the download speeds for the needs when its needed , and policy is configured to let specific users use those lines when in need of high download speeds.

[style="background-color: #0000ff;"]Backup[/style] one have 4 other lines which are actually cellular data routers and load balancing between those works perfectly fine.

Now to the issue/problem:

The request is to make ALL those failover load balanced lines, meaning that the place must have internet at all times, when dsl fails - cable ones should take place , when dsl AND cable fails , cellular lines should take place, the switch time between all those should be up to 10 minutes.till now i didnt managed to get good load balancing times , seems like because of the cellular delay its not working as intended when combined with regular lines (delay of cellular lines is 200+ ms higher)

What did i do till now:

tried to put all those into one load balancer , session based , and the problem is when one of the lines fails , fortigate still routes user machines thru the line which is fails , i set health check to different dns servers like google or other ones , it does the same, seems like the ping health-check isn't stable or something , and fortigate keeps bouncing users between different wan lines/IP's/ISP's, which makes it impossible for users to have stability.

How should i configure my fortigate to make it wan load balance 10 lines and failover them in the order : adsl-cable-cellular?

any help appreciated.