IPV 4

I'm sure your IT department is more knowledgeable about your VPN than I. The solution I use is for the retail market, not the corporate market like FontiClient.

I have to admit that I'm a bit baffled since IPV6 has been around for quite some time and most devices support it. I would have expected that FontiClient DID support IPV6 and that T-Mobile DIDN’T, but not the opposite. Life never lets up on surprising me. 

At the risk of asking a “Is it plugged in, is it turned on” type of question, is the FontiClient firmware/software up to date? Are they offering you the latest client? Lots of companies don't take maintenance contracts and the software can quickly become out of date.

Here are a couple articles that may offer more insight on how to fix this:

https://community.fortinet.com/t5/FortiClient/Technical-Tip-DNS-issue-with-FortiClient-SSL-VPN-when-IPv6-is/ta-p/197789

https://docs.fortinet.com/document/forticlient/7.0.0/new-features/186594/dual-stack-ipv4-and-ipv6-for-ssl-vpn-7-0-1

Let me know how it works out for you and your IT staff.