The problem is your Dell DA300 adapter (I assume you mean this one, as the DA301 does not exist). This adapter requires DisplayPort 1.3/1.4 in order to drive 4k at 60 Hz, otherwise it drops down to 30 Hz. This is detailed in the manual for the DA300.
The MacBook Pro 13" 2018 does not have hardware support for DisplayPort 1.4, but only provided DisplayPort 1.2. Only the 15" model has hardware support for that. Similarly you monitor only supports DisplayPort 1.2.
The monitor however does have support for HDMI 2.0, which is the minimum version necessary to support 4k at 60 Hz. Sadly, HDMI 2.0 is not supported on USB-C. As this adapter is USB-C, it won't be able to input a HDMI 2.0 signal. Had it been a Thunderbolt 3 adapter, that would have been a possibility - but it is not.
You might notice that the adapter is advertised as HDMI 2.0. The way this works is that the adapter takes in the DisplayPort signal over USB-C and converts that to HDMI 2.0 itself. Obviously as we've just established that the DisplayPort signal in this case does not support 4k @ 60 Hz, then neither will it support that when converted into HDMI.
In order to achieve 4k at 60 Hz, you can just use an ordinary cable (i.e. not an adapter) from the Thunderbolt 3 port on the MacBook Pro to the DisplayPort port on the monitor.