In the image, you can see that your signal values are as follows. RSRP: -115 RSQR: -13 SINR: -14.7 Signal Strenght: -71. In the last link I sent you, you can see the values for the 4G connection. Although you have a decent signal for 4G. The RSRP and SINR values are in the no signal and disconnect range. Maybe you can improve your signal value with a better antenna than the ones that came with the device.
The DLink device may get a slightly different configuration than the RUTX14 (Bands and APN), it is also possible that the DLink antenna gets a slightly better signal than the RUTX14. Is there any chance you could try that SIM on another device, maybe your phone? Then we can validate if there is good coverage in the area, at the moment it seems that it is not taking the optimal configuration for its best performance. Either because of the APN configuration or the band selected automatically.
At the moment, if the provider does not tell you which band to use. You can try selecting them manually to see which one can improve your connectivity. This can be done via Network→Mobile as above.