Hello,
Unfortunately, Teltonika devices do not measure VSWR.
You can have an indirect estimate based on either mobile or WiFi signal indicators in the WebUI, such as RSSI, SINR, RSRP.
The general rule of thumb is not to exceed 10m of coaxial cable, however, the actual length depends on the quality of the cable and its attenuation.
Best regards,
Žygimantas