Depends what you are trying to achieve. If the idea is to get more speed by having two connections, then no you can’t. You can’t have two routers connected to the same internet connection, as Liquorice points out.
However, you can have the ISP router connected to the internet and then it’s LAN providing the feed into the EWAN port on your own router, so you can effectively continue to use your own router. You may have seen this on the Internet, which where I think the confusion comes from.
Both would need to be on separate networks, say 192.168.0.x for your router and 192.168.1.x for the ISP’s, with your router using the ISP’s router as its gateway address. There are one or two other factors to take into account as well. It's not a simple setup.
I’ve used the same technique years ago, to run an ADSL only Billion 7800 through a Home Hub 5 when FTTC/VDSL first came out. Worked fine for 18 months, until I replaced the Billion with a Draytek. It’s do-able but probably beyond the average home user. (No offense intended).
