Hello @NETITCY
First of all: Each GX50 must be in the same company to form a Site-to-Site VPN
I would suggest you change the look of the network at the main office a bit.
The GX50 at the main office will be connected to the UTM firewall directly and your LAN will be managed by the GX50.
I suggest to change the LAN settings on the UTM to 192.168.0.1/32. The GX50 will pick up the IP from the UTM and it should be 192.168.0.2 on the WAN connection. Create a VLAN on the GX50, like 192.168.10.1/24, and connect your LAN to it.
Now you need to open UDP Ports 9350-9381 on the UTM for the GX50.
Once this is all set, you should be able to complete the setup by following this guide Meraki Go - Site to Site VPN - Cisco Meraki
If you need a single computer to access the VPN open UDP Ports 500, 1701 and 4500, those are used for L2TP/IPSec-VPN. Meraki Go - Client VPN Setup - Cisco Meraki
Hope it helps