Looks like the wifi subscription has been discontinued. From Premier Voice:
After spending at least thirty minutes today with our web support team and giving them your MileagePlus account, they concluded the wi-fi subscription option is no longer being offered at this time since they also could not find an appropriate link.
Their conclusion is reflected in the FAQs at the following link:
https://www.united.com/web/en-US/con...wifi/faqs.aspx
The second item in the second section notes: We currently don’t offer daily, weekly or monthly Wi-Fi Internet passes, but we may offer these options in the future.
An overview is at the following:
https://www.united.com/web/en-US/con...i/default.aspx
For the time being, wi-fi service then seems to be available as a pay-as-you-go option, and I am sorry for any inconvenience.