I am using thje B252 modem and am mucking about trying to improve my latency. I have noticed that when set to "Auto" or "4G 2300MHz only" I average ~20Mbps but when set to "4G only, no 2300MHz" I'm getting ~70Mbps with a handful topping out at ~100Mbps. Uploads are similarly boosted.
My questions are:
You can't improve your latency by changing your frequency, it won't make any difference.
Answers to your questions:
1. Leave it on Auto
2. No, you can't improve your latency (as mentioned above).
3. This just forces 2300mhz connectivity to Optus network. It may be that this frequency is less busy.