Were the IP ranges provided by the ISPs? If so, you are out of luck. These IPs are owned by the respective ISP and are reachable only thru that ISP.
To do what you are describing, the best way is to get an IP range of your own and a BGP autonomous system number. Then you arrange for the ISPs to negotiate BGP with your router. This will allow you to advertise your IPs thru both providers. There some fancy things you can do to have some IPs prefer one line and some the other line. Also, the main goal is to configure BGP to allow the secondary link/s to be a higher cost than the primary. When the primary link goes down, traffic will be routed to the higher cost link.
As you can see, this isn’t trivial. I am in the process of planning this for a public TV station and it’s already getting complicated with all of the involved parties. Good luck.