It looks like the router has told you it's limitation: max of 256 addresses, or a class C space. I'm not aware of anyway to change it. Others here may comment.
I point to and quote part of https://www.verizon.com/about/sites/default/files/fios-qgr-userguide140925.pdf
user's guide/manaul for that unit in question.
FREQUENTLY ASKED QUESTIONS10.1n/ HOW MANY COMPUTERS CAN BE CONNECTED THROUGH MY GATEWAY? Your Gateway is capable of 254 connections, but we recommend having no more than 45 connections. As the number of connections increase, the available speed for each computer decreases.
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