I have a BV6000, operating on Android 6.0 with build number BV6000_Blackview_V01_20161025.

I live in South Africa and have a Vodacom SIM for calling and another I use for data.

I recently traveled to the Republic of Congo and found that I could not connect to the local cellular networks, although my main SIM is enabled for international call roaming.

Is there any setting on the BV6000 that could have disabled international call roaming?

There is only one setting for enable/disable DATA roaming in the phone. If call roaming don't work you can check the frequencies in the other country or ask your carrier if roaming is blocked.