It's not "preferred network" but "preferred network type". Verizon are CDMA-based, so you want the list of options to include CDMA and EvDo (2G and 3G) and also LTE (4G). There's no harm in it including other types as well (useful if you ever travel abroad), but those are the minimum. My Pixel is by default set to "LTE/CMDA/EvDo/GSM/WCDMA (PRL)", which covers everything except some Chinese networks, but you may not have a single option which includes all of those.
If it already has the ones you need covered, leave it alone. Don't fiddle with stuff you don't need to. I mainly provided the instructions for that menu in case the previous person had been playing with it and didn't realise what they'd done: if you've not messed with that previously you shouldn't need to now, since the phone's Settings don't let you select options that should stop it working with your carrier, whereas in that menu you can do that, which is why playing with it was a possible cause of the problem.