I run Home Assistant, and one of the key things they emphasize there is that the ZigBee receiver should NOT be plugged directly into the computer hosting HA. It should, instead, be plugged into a USB extension cable and placed away from the computer.
This is true for general ZigBee hubs and I'm sure it holds true for your SmartThings hub, as well. I've got my hub sitting on the top of an 8' tall cabinet (we have tall ceilings) and once I learned how to get the bulbs configured, I haven't had any issues with my Sengled bulbs dropping off the network. (I have different bulbs, but they are from Sengled.)
Another thing emphasized in the HA setup instructions is that the hub should not be plugged into a USB 3.0 (or newer) port, but should be on 2.0 only. Despite the backwards compatibility, it seems that there is some issue in the driver stacks (this advice applies to HA hosted on Windows, Mac, or Linux) that can cause ZigBee to have issues.
Since you mention that range is an issue, a quick internet search for "smartthings range extender" turned up this thread on the SmartThings support page. It's got links to Samsung docs on how to add hubs as range extenders to improve reception at distant locations. I'm sure that there are other pages available that will give you a description of how to do it. It does appear, though, that there are different options based on what version of the ST hub you currently have, so be sure to pay attention to that.