Smart watches support work in 2G mobile network and already have the Internet settings (APN) for all mobile operators.
Check the balance and the Internet of SIM card. Check the operation in 2G mobile network (you can check this information with your mobile operator).
If the Internet on the SIM card is connected, the balance is positive, but the Internet still does not work - turn on the smart watch, send SMS message with the APN settings to the following format:
APN#port#SIM ID#APN name#username#userpwd#ip
For example, Motiv mobile operator - APN#0#25035#inet.ycc.ru#motiv#motiv#0
If one of the items should be empty, need to set 0 (for example, Motiv operator this are items "port" and "ip")
How to change APN settings - https://elarihelp.freshdesk.com/a/solutions/articles/44001774203/en