if the router is in bridge mode, then it functions as a modem.
so the next router, in this case, AR300M, will not be in doing double-nat.

if your AR300M wan’s port is getting a private ip addresss from the isp device, then the isp device is not in bridge mode.

the main point in having the isp device be in bridge mode, is to prevent double-nat, for example, which makes problems for VOIP.