Can you ping the devices across subnets? Try both IP & hostname - If you can ping IP but not name, check DNS settings. Be sure your internal DNS servers are listed under Security & SD-WAN > Client VPN under Custom nameservers.
If both fail, make sure you have the routes properly configured. Are your VLANs configured under Security & SD-WAN > Addressing & VLANs?
You can also look under Security & SD-WAN > Route table to see what routes are available to your VPN users. If the necessary routes aren't there, there will be communication problems. If using a separate router to traverse VLANs, there either has to be a route on the MX to that router or the routes also need to be in the MX configuration.