Yes, of course. In this way, you can save a lot of money as long as it handles your 100Mbps Internet speed plan. Indeed, you don't need to buy any modern router with the greatest features and technologies unless you need them.   However, make sure the older router supports at least 802.11ac wireless standard and offers dual-band frequency. That's because most modern routers are WiFI-5 compatible; going with the older wireless standards, such as 802.11n and 802.11g, would not be a smart decision even though they handle your 100Mbps plan.