This seems like it should be simple. However, modem-routers don't always have their modem speeds mentioned in their specifications.
You find out by looking up the DSL standards that the modem supports. DSL standards give you a theoretical maximum speed the link can offer. Note that there are different variants of DSL, such as ADSL and VDSL.
Of course, in reality, those speeds may be lower due to various influence factors, most importantly the distance between your modem and the exchange. Also, the carrier may artifically lower your connection speed.