Some things to check…
COM PORT settings.
Check to see there are no conflicts. ie, 2 devices trying to use the same COM.
Turn down your baud rate to the actual speed of the modem…(this won’t solve a h/w install issue though).
Ensure when you removed the previous instance of that device, it actually got removed. Boot into SAFE MODE, go into DEVICE MANAGER and check to see what ‘orphaned’ devices are listed….remove them all. You may see duplicates of alot of devices as well.