Adding Standalone Unlimited Data To Legacy Gigabit x2 Plan

I currently have a legacy Gigabit x2 plan for $85 per month, with speeds up to 2100 Mbps download and 300 Mbps upload. I also use my own modem.

I would appreciate help adding only the standalone Unlimited Data option for $30 per month to my existing plan.

During an earlier attempt, the approval page unexpectedly listed: “Removed From Your Plan — Gigabit x2 — Cancelled”
I was not comfortable approving that order.

Please help me prepare an add-on-only order that keeps the following unchanged:
- My legacy Gigabit x2 plan
- My current $85 base monthly price
- My current download and upload speed tier
- My customer-owned modem
- My other existing services and account terms

Thanks!