The provisioning menu on the 525G is very similar to other Linksys/Cisco devices, such as their ATA's. There are a number of examples of manually provisioning devices such as the SPA2102, SPA3102 (FXS port), and the like, on the forum.
If you continue to have issues getting it working, start a new post, on the topic of manually provisioning of that set. I should be able help you get the basics working as I have about 5 of those sets that have all been manually provisioned. Actually one of my favourite sets, given the built-in Wifi , bluetooth, colour screen and the fact it will register with multiple servers..
The 3CX Activity log will let you know if the PBX is getting registration attempts, and if some parameters are not correct. Too many incorrect attempts however, may cause the IP to be blacklisted, just be aware.