Hi,
Here, Be honest, you will need a load balancer where you can decide or take a decision for bandwidth sharing based on the bandwidth available from the ISP end.
Are you using the default route for all ISPs or it is BGP or other routing protocol? If you are using default route or dynamic routing then load balancing is possible (depends on dynamic routing). But I hope you need some type of NATing and where is it configured?
If you will share a network diagram with us then it will easy to explain the same.
Regards,
Deepak Kumar,
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