Hi Ex0r
Internal networking equipment can be a bit tricky
Most networks run two domains an internal domain (e.g. domain.company.local) and an external domain company.com
If you run an internal domain that is the same as your external domain you are in luck. e.g internal.company.com. You can request a certificate for aruba.internal.company.com and use DNS verification to get the certificate issued. You can then install that certificate on Aruba APs aps and update your DNS records.
Lets Encrypt works well for the external domains used in emails and websites but like most CAs don’t do well in internal domains.
There are a couple of options
A) If you are running microsoft and have microsoft AD Certificate Services enabled you can issue your own certificate eg. arubalogon.compay.local and make sure DNS records point to the access point.
B) You can create a self signed certificate and install that. Once again the domain name might be arubalogon.compay.local
C) You can use a Lets Encrypt Certificate. The challenge here is that the internet will not be able to resolve arubalogon.compay.local so you may need to do some magic to make that happen