Can't access certain subdomain website? DNS NSEC3 issue?

Edited on 2017-10-25. Original question was misleading.

We have a website running on the subdomain http://admin.gigantisch.nl/. Ever since the gigantisch.nl domain changed IP addresses, we have been having trouble accessing the admin subdomain on various devices and networks. Experiments and diagnostic tools seem to indicate that this is a DNS issue.

We had been able to access the site successfully multiple times in the past days, but this seems due to cached DNS records on our various devices (including routers). It might also be due to cached records by both our ISP's DNS servers and Google's public DNS servers.

When run through the online diagnostics tool: http://dnsviz.net/d/admin.gigantisch.nl/dnssec/, it indicates there is an issue with the NSEC3 record. I have no idea what this record is, or how to manage it. Our domain registrar's DNS zone editor panel has nothing regarding NSEC records.

Something that makes all this more baffling, is that another of our websites running on the subdomain: http://backoffice.gigantisch.nl/, has none of these issues.

ORIGINAL QUESTION:

Here in this office, we have a site on the subdomain of our main website we can access through our wifi connections, but not through our wired network. I can also access it on my phone using cellular data.

The address of the site is: http://admin.gigantisch.nl/

The entire domain recently changed IP addresses, so it's likely there is some sort of DNS problem here. When verified with Google's DNS tool (https://dns.google.com/query?name=admin.gigantisch.nl&type=CNAME&dnssec=true), it states under "comment": "DNSSEC validation failure. Please check http://dnsviz.net/d/admin.gigantisch.nl/dnssec/."
That tool, gives us the following errors:

"NSEC3 proving non-existence of admin.gigantisch.nl/A: The NSEC3 RR covers the wildcard itself (*.gigantisch.nl), indicating that it doesn't exist.
NSEC3 proving non-existence of admin.gigantisch.nl/A: The NSEC3 RR covers the wildcard itself (*.gigantisch.nl), indicating that it doesn't exist."

Which at least indicates there is a DNS problem, I guess?

What's also strange, we have none of these problems with our other site, http://backoffice.gigantisch.nl. The DNSViz tool doesn't point out any error or warning for this subdomain either.

I was thinking maybe the old DNS records for the gigantisch.nl domain were cached by the wired router, but I gave it a soft reboot without avail.

The DNS servers for both the wired network as the wireless network appear to be set to Google's servers, 8.8.8.8 and 8.8.4.4.

There is a DNS A record setup for *.gigantisch.nl, but for neither of the subdomains.

Also, when I run Windows 10's Network Diagnostics on my client, I get: "Your DNS server might be unavailable". I don't seem to be experiencing any other connection issues though, so this might be unrelated to the problem at hand.

Ideas?

Edit: When I setup my local connection to use my ISP's default DNS servers, 62.179.104.196 and 213.46.228.196, I can access http://admin.gigantisch.nl/ without issue. When I setup the router to use those DNS servers however, I again can't access it, i.e., Chrome gives me the error ERR_NAME_RESOLUTION_FAILED.

Edit 2: The devices on the wireless network I tried, seemed to have cached the old DNS resolution of the (sub)domain name. Loading it on one of the positively tested mobile devices in an incognito tab on Chrome for example, yields: "this site can't be reached", accompanied by DNS_PROBE_FINISHED_NXDOMAIN. (By contast, Chrome on a Windows client on the wired network gives: ERR_NAME_RESOLUTION_FAILED.)