First, you need to confirm compatibility with dd-wrt:
Then, you need to configure as per:
https://www.zong.com.pk/value-added-services/mobile-internet/internet-settings-guide/internet-settings-guide (try the settings for android phones first)
Usually, the device shows up as cdrom to install drivers and/or related software when plugged in and needs the mode switched to modem mode to work solely as a modem! Sometimes, it might be necessary to do so by sending particular AT command(s) to the device, but, if the device (vid:pid) is on the list, it should work well without doing anything extra!