The max memory speed supported for that board is listed at 2400mhz according to the Asus website. You might need to upgrade your bios to go beyond 2133
https://www.asus.com/us/Motherboards/ROG-STRIX-B250F-GAMING/specifications/
"Due to Intel® chipset limitation, DDR4 2400MHz memory frequency is only supported by 7th Generation Intel® processors. Higher memory modules will run at the maximum transfer rate of DDR4 2400MHz.
** Due to Intel® chipset limitation, DDR4 2133MHz and higher memory modules on 6th Generation Intel® processors will run at the maximum transfer rate of DDR4 2133MHz."