Adding DHCP Option 66 should not prevent devices getting an IP address. If you have a PC, Mac, smartphone or other device that is able to get an IP address, then the problem is likely with the GS-GXP.

I think the string should be tftp://a.b.c.d or http://a.b.c.d (may also have a suffix) that your provider/administrator should advise. I use a Grandstream ATA for VoIP, but definitely not an expert at this.

I do not work for and I do not have formal association with GL.iNet