security – Making a domain more trustworthy to corporate networks so that it won’t be blocked by their filters

I developed a web app for a customer which is made in basic PHP/MySQL, pretty normal site, lighter than WordPress, and when my customer went to his client to sell the app the domain was blocked by the internal router.

He was first on the guest-WiFi, domain was blocked, but then they tried to plug his computer directly to the router with a cable, domain was still blocked. His clients are big factories, and I know those tends to have higher security on their local network.

I’m looking for solutions to make his domain trustworthy so this doesn’t happen again when he presents what he has to sell. My research was not concluding enough for me as it just gives me examples to make the website LOOK more trustworthy or it’s people having problems with their home network. Hence why I’m here looking for help.

Aside from manually white-listing his domain on each factories’ network he visit, is there a solution?

The only thing I figured out so far is that it may be because his domain is young and only renewed for 1 year at a time, we suggested him to renew for 5 years at a time, but I fear that this won’t be enough. Is there something to do in the DNS?

I sadly don’t have any information of the factory’s network setup and protections. I’m more looking for a solution that would work everywhere, if possible. Like Facebook will never be blocked anywhere because of its authority and trustworthiness, but how do I get a domain to that level of acceptance by security?

Domain name: