MikroTik

Good day everyone!

Okay, so I previously had two routers, which I now replaced with a single CCR1009.

I have three public IP's, all assigned to the same interface, and I have port forwarding figured out from specific WAN IP's.

The issue I have is forcing specific subnets to use specific WAN IP's as their gateways to the outside world.

For example, I would like to force:

Subnet1: 172.16.101.0/24 to use (for example) 1.2.3.4;
Subnet 2: 172.16.102.0/24 to use (for example) 5.6.7.8;

Also some specific devices on other subnets to use a specific WAN IP.

I've tried a few different ways of making this work, but I've been unable to up until now. I saw a post about adding mangle rules and marking the traffic, but the moment I do that it breaks access between my subnets.

Could anyone help please?

Have you tried specifyng src-address in nat rule? (also you can try src-address-list in advanced tab for specific IPs, making your own list in firewall-address lists )
Something like:

/ip firewall add action=src-nat chain=src-nat out-interface=ether1-wan src-address=172.16.101.0/24 to-addresses=1.2.3.4
/ip firewall add action=src-nat chain=src-nat out-interface=ether1-wan src-address=172.16.102.0/24 to-addresses=5.6.7.8

One has to be careful of ensuring the requirements are understood.
Can you clarify that you are addressing only traffic ORIGINATING on the router heading outbound.
In other words you dont run servers where people are originating traffic external to the router heading for your LAN network.

If its the simple case, then for whole subnets its easy.
For individual IPs, suggest you try to put them in a subnet together to make life easy.
If you cannot, then you have to mangle traffic, all very doable but adds a layer of complexity.