EC25AUXGAR08A03M1G setting to Band 28

Hi, I have a EC25AUXGAR08A03M1G and it is my understanding I should be able to get band 28 (700) on the Telstra network in Australia.
I have flashed the modem with open Wrt to give me access to AT commands.
I cannot see band 28 under any of the LTE settings
Would you please provide me with the correct AT command to set the modem to band 28?
I have also noticed roaming is enabled
Would you also please provide the correct AT command to turn this off?
The only reason I flashed the modem in the first place was my download speeds would start off at 30+ Mbs and then drop down to about 1Mbs making the modem useless
Thank you for taking the time to help

1.AT+QCFG=“band”,0,8000000,0 Band 28 can be locked by this command
2.Any band can be restored by modifying the parameters

Thank you [Jouni.yang]
You have been very helpful
I have set the modem AT+QCFG=“band”,0,8000015,0
This should give me bands 1,3,5 and 28 which are the Telstra towers in SA
Thank you for your assistance
I seem to drop 20Mbps between ethernet and wireless
is there a way of lessening the losses?
Thanks again