New ADSL Modem

If your DSL router was supplied by o2 and is defective, it will replaced by a working one for free by o2. 

If your internet user name provided by o2 ends with @s93.bbi-o2.de you are using a VDSL2 connection and no ADSL device may be used. Apart from that, I would generally advise against buying ADSL equipment yet.

If buying new, I would recommend an AVM Fritzbox 7530/7530AX/7590. 5-year manufacturer warranty and the devices are guaranteed to work on o2 DSL connections including telephony. 

If necessary, you can also consider a cheaper used AVM Fritzbox, e.g. Fritzbox 7490, 7560, depending on the desired equipment. Look for used devices e.g. at ebay-kleinanzeigen.de
Please note that all Fritzboxes mentioned are a combination of VDSL2 modem and WLAN router. They cannot be used as a pure modem and models with firmware <7.20 have only German as menu language. From firmware version 7.20 on, English can also be set as the menu language.