The FiOS router already has two antennas; one is a small chip inside of the router with metal pathways on it, and the other antenna would be the one attached to the rear of the router. What you can do is purchase a high gain antenna, or a directional antenna if the router is in the corner of your home and see if that works since the rear antenna is detachable. What I would also suggest doing if the antenna isn't an option would be to buy another cheap router, or preferably an access point to extend the wireless range. I'm not a huge fan of Wireless Extenders simply due to the way many of them operate; they cut your speed in half and are limited by the signal they are receiving from the AP you wish to repeat.
What can be done if you don't want to run Ethernet to the location where the second AP would be located, is to simply order another ActionTec MI424WR router from the Internet (they can be bought used online for usually $30), and using a nearby Coaxial jack, set up a MoCa link between your primary router and the second ActionTec router you're using as an access point (and possibly for additional ethernet ports). This method requires some additional configuring, but it does work. There is always the option of using an Access Point with a MoCa Bridge, but this is often a more expensive but easier to set up solution compared to using someone's old ActionTec. Same goes with using Powerline Adapters.
If you don't want to wire up anything or buy anything, the least you can do is to try setting up your current FiOS router to use another channel, preferably one not used by your neighbors. A useful tool called InSSIDer made by Metageek can allow you to see where your neighbors are broadcasting at and allow you to pick which channel you should run your router on. What also helps too, since wireless is an animal that works better in certain locations (Think: TV Antennas), would be to simply relocate your router to another section of the house if possible. Sometimes even moving it to another part of the room it's located in or the router's positioning can do wonders. If you don't have any PC currently connected via Ethernet to the ActionTec, this is a nice possibility. Just keep in mind, if you do move the router it should be in an area where a majority of the wireless usage stays in a vertical path through the walls. Wireless does not like traveling up or down. With that being said, Basements and Attics are terrible places for a wireless access point.
