HG533 UTMS username/password

Hi

I'm trying to connect an E1550 dongle with a HG533 router, the dongle is recognised on the Status>WAN>UMTS page. However when I come to configure the APN settings on the Basic>WAN>ttyUSB0 page, I get the following error message:

"1 error message(s).

The error message(s) is(are) as follows:

Password length should be between 1 and 32 alphanumeric characters and special characters."

However the ISP (ID Mobile) says to leave username/password blank?

https://www.idmobile.co.uk/help-and-advice/get-your-...

Any ideas?

TIA