2 separate sip trunks working simultaneously

I have a server computer with 4 ethernet interfaces that I’ve been using as an Asterisk server. I have 2 sip trunks that each occupy 1 ethernet interface because they’re from separate sip providers. They work independently, able to call and receive - however because the server only has 1 default gateway, I can’t make them work together at the same time. Whatever the default gateway is, only that sip trunk works and I can’t ping the other.
I’m sorry if I mixed up the terms but hope you understood what I’m trying to do. Any idea on how to accomplish this?

You need to configure explicit routes to the sub-network for each ITSP However note that configurations with invalid multihoming, like this, are always fragile.