Do you have a Service Activation Notice or Modification Notice from your carrier?
I'm not sure I would want to apply configuration to the SRX without confirmation on how the carrier as allocated and setup the added 121.45.21.128/25 on your service.
Assuming they are using method 1 adding the prefix as a layer two on the same interface, your configuration would need the following changes.
interface:
You are using the gateway address, this would be the address on the carrier interface, yours would be the next available one:
NOT: 121.45.21.129/25
Use: 121.45.21.130/25
Route:
You don't need a second default route there will be no change here.
But if you did you would use 121.45.21.129 as the next hop 128 is the subnet address.
NAT
For addresses in a layer 2 domain you will also need to add proxy arp to the interface in that subnet. You won't need to do that if the subnet is routed via method 2.
Check out the examples of all the options in detail here for source, destination, static and double nat.
https://kb.juniper.net/library/CUSTOMERSERVICE/technotes/Junos_NAT_Examples.pdf