How is the monitor connected to the GPU ?
ASUS AMD RX470 supports:
DVI Output : Yes x 2 (Native) (DVI-D)
HDMI Output : Yes x 1 (Native) (HDMI 2.0)
Display Port : Yes x 1 (Native) (Regular DP)
HDCP Support : Yes
I have tried DVI-D outputs 1 & 2 and the HDMI outputs to a DELL DELL-E248WFP which has 1x DVI and x1 VGA ports. The HDMI on RX470 was connected via HDMI to DVI converter. Note the RX470 only supports DVI-D and not DVI-I. I could not use the RX470 Display Port output as the monitor does not support it and neither do I have a converter for it. If RX470 had supported DVI-I, I could have used a DVI-I to VGA adaptor to test as well but it does not. My assumption is the VGA VESA modes can be transmitted via the DVI-D.