Cisco SPA504G: can't get IP address at work but can at home...

I'm new to RC and setting up service both at home and my own small business.

I'm having issues at work setting up the 504G phones. I've tried setting up 2 phones at work and both now remain on "Initializing Network". (When I go to Setup > 9 Network, I see 0.0.0.0 so I know that they are not getting local IP addresses issued.) Interestingly, at work, I did successfully set up 1 504G phone but when I tried setting up the 2nd 504G phone, the 1st stopped working.

At home, I set up 1 504G phone and it works fine.

At home AND at work, I'm using Comcast for Internet. The only difference as far as I can tell is this:

at home, I use a Comcast-issued cable modem: TECHNICOLOR tc8305c
at work, I also use a Comcast-issued cable modem: Netgear CG3000DCR

Any ideas on how I should go about troubleshooting?
I've read a bit and heard I should just buy my own modem for work - something like the Cisco DPC3008, or I could pay for a static IP address ($19.95 per month). Buying my own modem would be cheaper in the long run.

Thanks.