I am sorry.
Due to core fabric GPIO architecture changes, Triple Speed Ethernet IP Core with RGMII mode has not been able to close timing in Quartus® Prime TimeQuest for Arria 10, Cylone 10 GX and Stratix 10 devices. Therefore, Triple Speed Ethernet IP Core is not supported for RGMII Mode in Arria 10, Cyclone 10 GX and Stratix 10 devices as there are risks of hitting failures in the actual hardware.
There is no workaround for RGMII mode. Please use other interfaces such as GMII or SGMII with Triple Speed Ethernet IP core.
Sorry, the problem right now is on the silicon limitation where it's hard to close timing with RGMII interface interface with FPGA IO.
So, the issue is not about the conversion anymore but we simply can't support RGMII interface usage on Arria 10 FPGA IO pins.
This is silicon hardware limitation, not about IP core usage.