Two offices with EdgeRouters connected via VLAN and not working Gigaset

Hello guys,

I'll preface this with saying that I'm completely new in this field and not really sure what this all means, please bare with me here. I've been given a task to resolve this issue at my work but since I have no network expierence (working in web dev) it's been a struggle and I'm kinda out of my field, would love some help.

We have 2 small offices that are in different physical locations that are connected via VLAN which was configured by their ISP. Both of the locations use EdgeRouters (8 port and Lite). Everything other than Gigaset phone works perfectly fine (RDP, NAS etc) but when it comes to the phone it's got the signal but we cannot actually hear anything on neither sides. From the main location we can ping the Gigaset but cannot access the web-gui. From the guy who actually made the VLAN I heard that it's NAT related problem but it didn't really help me solve it as I'm clearly out of my field.

I assume this is not enough information to be any specific in answers but just general advices would be a tremendous help. I will also try to answer all the questions. Thank you guys.