Verizon used to use coax to connect the router to the ONT for the WAN connection. It is only good for speeds up to 100Mbps. For higher speeds, an Ethernet connection is required.
Further, the older ONTs are only good for up 75 or 100 Mbps. If you have one of those, Verizon will need to come and replace it a new model. You can't do that yourself. The new ONTs are black and about the size of a decent router. If you don't have one of those, you need an ONT upgrade. If you're not sure, post a picture of your ONT.
If you have a new ONT, you probably can avoid the upgrade fee by switching to Ethernet on your own. Run an Ethernet cable from the ONT to the router's WAN port, then contact Verizon and have the switch the WAN connection from MoCA (coax) to Ethernet. Takes about 5 minutes and they won't charge for it. After the change is made and propagates through their systems, you should be able to upgrade speeds without any fees.
Even if you have an older ONT, switching to Ethernet now will simplify the upgrade process. Depending on where the router and ONT are located, you may want to run the Ethernet cable through walls. Verizon won't do that, so you can get a neater install if you do it yourself.