Can't reach multi-homed Ubuntu from different subnet
The client needs to know the route to the 10.100.100.0/24 network as well.
so you either need to add a route on the client
ip r a 10.100.100.0/24 via 10.100.100.1 dev eth0
or you need to add the route on the default gateway the client is using.
you can think of it this way: the client needs to know how to reach 10.100.100.21, if it doesnt, it will try the default gateway, if the gateway doesn't know, you are out of luck.