People have reproted at times that different speeds have come and gone.
What area are you located?
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More than likely, this is because you're on 75/75 or lower and have an older ONT. Verizon will require you to upgrade to 300Mbps, which will warrant a tech roll to replace the ONT. At that point, you'll be able to downgrade to 150/150. Verizon won't offer 300M or 500M packages to you if the area lacks the capacity for those tiers (Only so many people are permitted to buy those tiers currently in a specific area).