Hello Pavel
2650 XM supports a wide range of WAN modules.. what is the local lead from the ISP ? If it is ethernet, you can buy a NM-1E or any of that kind.. the router comes by default with 1 FE which would connect to the LAN. It can support 1 NM and 2 WIC cards.. and you can choose what you want from a wide range of WAN modules , given in the URL below:
http://cisco.com/en/US/prod/collateral/routers/ps259/product_data_sheet0900aecd800fa5be.html
hope this helps. all the best. rate replies if found useful.
Raj