Cannot add unlimited data option to plan

Hi,

I'm using 1Gig plan with my own modem. I want to add unlimited data option to my plan. I tried activating it with this link https://www.xfinity.com/buy/plan/checkout/XFINUDAT but it always ends with this error

Temporarily unavailable

The product you’re interested in isn’t available for purchase at this time.

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Please help! Thanks in advance.