G2/3 Incremental

I have a CNC router with a ECS 1801 controller. When I try to use G02 it gives the error radius incorrectly programed. The fault is that the I.. J.. is expressed as absolute by default. The controller can be changed to incremental, anyone know how? I have tried G91.1 but it doesn’t recognize it.
I have other machines, post processors and programs that I would like to bring it into line with that run in G90 but the I.. J.. is incremental.
Any suggestions and advice would be welcome.