public IP routing question with two ISPs and four Subnets

Hi,

i have a setup and want to ask if someone has an idea about it ;)

We have two ISPs and each ISP offers two Subnets. Currently there is router construction in front of the old firewall.

ISP-A: 200.11.11.0/29 (ISP Gateway 200.11.11.1)

secondary IP Subnet: 88.88.88.0/28

old router WAN IP: 200.11.11.2

ISP-B: 100.10.10.0/29 (ISP Gateway 100.10.10.1)

secondary IP Subnet: 195.22.22.0/28

old router WAN IP: 100.10.10.2

It is easy to get everything in the primary IP network running. But the secondary subnet have to be routed to wich adress and how does the ISP router know the way back?

How can i make the secondary IP adresses get work, if i have only a sophos UTM in the new setup.

At the end i would like to use all ISPs and all subnets

Thanks fr helping