The picture doesn't make sense until you label the devices with the red arrows.
You shouldn't have to do anything to make the 2 networks communicate if both subnets are defined on the same router. If they aren't talking, you are blocking it.
You don't need to add any routes, they are already there as connected routes (ADC in /ip route print). Again, given that these subnets are on the same router.
I'm continually amazed at how bad advice propagates in this forum regarding 2 subnets talking to each other attached to the same router. Nothing special needs to be done.
Read the OP description again! He has a static network and is attaching devices that have static ip’s that are outside the scope of his network. He does not have multiple networks. I am truly amazed at how many give advise that truly does not read the full description, but only looks at the pretty pictures.
If the router is setup to use the 192.168.88.0/24 subnet and the device has a different subnet and he plugs it into his router. He now has two subnets on his router except that the router only knows of ONE of those subnets and what to do with traffic from/to that subnet. So the answer is setting up the device so it can talk to the router (i.e. reachable from the router, can ping the router) and then making the LAN subnet talk to it. So again, if you want to plug devices with different subnets and IP ranges into the router and have that router actual *route* that traffic, you need to set those subnets up.
So we do know what the OP is going for and that is why we are tying to figure out, are these other subnets programmed on the router? If not, then they need to be. If they are then they are not configured properly and routes are probably missing.