I recommend you jump on the phone, and call customer support at Verizon.
The "stores" are the least technically competent.
Regarding if the 5g card will work? It is my guess that it would actually perform 4G LTE properly in areas with no 5G. The question is what would it do in locations 5G was available. Possibly if you selected "4G only" on the LS, it would work as expected. There will soon be no fallback to 3G.