The relationships between the City Region authorities and the local authorities are not straightforward, and my understanding may be mistaken.
The funding for the airport is coming from the South Yorkshire Mayoral Combined Authority (the city region authority) and the Board of that organisation comprises of the directly-elected Mayor and also the leaders of Doncaster, Sheffield, Rotherham, and Barnsley councils. The leader of Doncaster council is the directly-elected mayor Ros Jones, who chooses the council's cabinet, even though the majority of Doncaster's councillors are Reform. She got in on a manifesto of getting the airport re-opened.
In that regard, what Reform councillors may or may not think is largely irrelevant- it isn't Doncaster City Council's money, it is the SYMCA's money.
The SYMCA gets most of its money from central government, so again what Doncaster City Council's councillors think about this is largely irrelevant. However the SYMCA does get a chunk of money from the local authorities via a transport precept levied on the local authorities, and that's where the local authorities do get some sway. My understanding is that they can refuse to approve the precept if they disagree with how it is being spent- that's something the likes of Bromley and Stockport councils have used before as a negotiating tool.