How to increase bandwidth from 128kbps to 256kbps on NM-8S?

I am new to this forum but been a member for a while and this is actually my first question. Thank you all in advance for any contributions.

I have an NM-8S module, it has 8 Low-speed serial(sync/async) interfaces on the 2811 ISR. All these ports are hard wired to 128kbps aggregate bandwidth. Is it possible to change the bandwidth on any one of these ports to another value in any way, maybe through some terminal configuration. One of these WAN interfaces is the Internet gateway to our ISP and we are supposed to be getting 256kbps through it. The question of buying another module is out.