Problem with Digital Display Unit over PMP450i

We use PMP450i to connect small offices to our Municipal Network.
Generally all of our applications work well and we treat wireless as an alternative to fiber, for sites that don’t need a lot of bandwidth.
But our Digital Display Units (DDUs) don’t work. These are Android based units that just need to connect to a web site and download data. The DDUs work when sites are connected over fiber rather than fixed wireless. And a laptop that uses the same fixed wireless connection is able to connect to the web site. Everywhere that we try it (two locations in the field and a mockup / test site) we have the same problem. There’s just something about the combination of DDUs and PMP450i that doesn’t work well together. Has anyone seen anything similar? Any suggestions on things we should check?

Do you use DHCP, and if so, are the DDUs receiving the correct network configuration? Are there any network diagnostic tools on the DDUs themselves?