Got our prepaid 4510L fired back up today to use at a vacation home (that was another whole story, but it's "sort of" working now).
What we're seeing is that on Windows laptops, the web is very hit & miss - some sites come back with DNS failures or site not found. Others seem to work fine. And no real pattern that I can see to what works and what doesn't.
BUT, when I fire up my VPN, suddenly everything is working fine. What kind of IP nat / firewall strangeness might Verizon be doing with these units such that the web is so flaky unless we're on a VPN?
VZW does all sorts of crazy things with thier NAT firewall. It is a very well known and documented limitation of the new LTE/SIM card network.
As you already mentioned VPN's are the only known and realistic work around to those limitations. Otherwise you would have to pay several hundered dollars for a static IP address without NAT services. My suggestion is to keep using your VPN as often as possible, especially if you hit these limitations on a frequent basis.