I don't believe you'll be able to sandwich the OBi202 in Bridge between your cable modem and your existing computer or router.
In Bridge mode, the OBi202 is effectively a 3-port Ethernet switch, with one of those ports connected to the internal workings of the OBi. Both the OBi202 and your existing computer or router need to obtain an IP address, but your ISP likely only allows a single IP address to be allocated through your cable modem.
Consequently, you'll need to connect the OBi202 in Bridge mode and your computer to the LAN ports on an existing router that's connected to your cable modem in order for all of your devices to share the single IP address that's available from your ISP.
To accomplsih what you set out to do, you'd need to purchase a second IP address from your ISP, assuming they offer that capability.
Note: Once a cable modem issues an IP address to a particular device, it typically remembers that device's MAC address and won't issue an IP address to any other device without the cable modem being power-cycled. This means that if you disconnect your computer in order to see if the OBi202 will work by iself, you must power-cycle the cable modem before the OBi202 will be able to obtain the one available IP address.