Hello,
Please answer the following questions in order to help better understand the situation:
1. Are your devices that are not able to resolve rms.teltonika.lt have Internet access?
2. What DNS server they are using?
In the meantime, I would suggest using an IP address instead of a domain name. The RMS service can be accessible via 18.196.62.30 IP address. It can be specified in the System -> Administration -> RMS WebUI page.