DNS not resolving to externally when use the DC in Azure

Hi, we have 6 Domain Controllers in total, 4 on-prem and 2 in Azure. 4 of our DCs are the DNS servers as well (2 on prem and 2 Azure DCs). Currently, we are using the on-prem DNS as the primary and secondary for our on-prem servers. For our servers in Azure (we have hub & spoke topology where we have 2 of our DCs in the Hub acting as the DNS as well), these servers in Azure are using on-prem DNS as primary and Azure DC DNS as secondary.

We would like to change the primary and Secondary DNS for our Azure servers to be using the Azure DCs (DNS) both as primary and secondary. When we do a test on our test vnet, the servers can resolve the internal clients but times out when try to resolve externally (such as google.co.uk / bbc.co.uk etc). After changing the primary and secondary DNS from on-prem to Azure DNS, we can also see it breaks the site recovery replication that we have in place between our region A-B. Are we missing anything that needs to be done before we can make this option to work without any issue?