Cannot sign in to Skype for Business T46G

[WerVermaak2]

We are using Skype for Business on premises server. The Firmware license on the T46G was successfully installed.

Current FW: 28.8.0.34
SFB License: Active

We have tried to sign in several times and only receive the message 'Register Failed'.

Login address: xxxx.xxxx@domain.co.za
Register name: domain\username
Password: xxxxx

Is there anything specific we need to change on our Edge or should I point the phone somewhere to autodiscover the information?

We are not using autoprovisioning to mass deploy as we only use four units.

Any feedback / assistance would be appreciated. Been struggling with this for days now.

[WerVermaak2]

SOLVED

Managed to find the problem.

Under SECURITY settings > DISABLE > Only accept trusted certificates.

>REBOOT > Sign in > User registration successful

Such a small thing can cause such a big headache.