Connecting to a typical DB-9 will likely fry your router, as most computers and USB adapters with DB-9 connectors work either at TTL levels (5 V), or RS232 levels (nominally +/- 15 V). The latter will almost certainly fry your SoC's serial port at best, potentially your entire SoC. Even TTL levels can be deadly to SoCs.
Only use an adapter that uses 3.3 V logic levels. Many need to be configured with a jumper or solder bridge to set 3.3 V. Others are TTL- or RS232-only and are not appropriate.
Do not connect +3.3V, V+, Vcc, Vss, or the like between the router and the serial adapter; RX, TX, and ground are sufficient.