Why external 4K Display will only display on 1920x1080?

I got a 28' UHD 4K monitor today (Acer Nitro VG280K), the tech spec does claim max resolution 3840x2160. However, when I connect my old MacBook Pro (late 2012) with HDMI cable, it will only display FHD (1920x1080), which is very disappointing.

• This old MacBook pro (late 2012) does have 2 MDP and 1 HDMI, I'm assuming the 2 MDP should be Thunderbolt 2
• The VG280K have 2 HDMI 2.0 port and 1 full size DP (DP1.2) port, I'm using the HDMI cable coming with the monitor.
• I tried to adjust the resolution by going to "System Preferences"->"Display". For the built in Retina display, when I choose "Scaled" instead of "Default for display", I can see there are 4 little squares. However, for VG280K, when I choose "Scaled", it only shows 4 options (refer to screenshot), none is UHD

Also this is the System Report->Graphic Display section, VG280K is marked as 1920x1080.

Really out of idea now. Do I need a better HDMI cable? Would a MDP->DP cable do the trick? Any advice is appreciated!