A router provided by an ISP is not your hardware, thus any network behind it is by definition not controlled by you. There have been numerous cases where they have backdoors or known admin passwords. In cases where there is a wire type transition (for example incoming over coax or fiber) it might be necessary to use it though. Same if it is necessary due to your contract.
In my cases I always turn off the wireless antennas and switch it to bridge mode, then place my own router/firewall device behind it.
Edit: still learning to spell.