Hey Francisco!
I’m glad to read that, we are here to help! 
In the loop of the code I can see that you have assigned the timestamp instead of the context as it’s mentioned in the comments of the code. Based on this, what do you desire to assign a context or timestamp?
Now, based on what you desire to sent you should follow the next points:
Ubidots::add(char *variable_label, double value, char *ctext)
Example:
sprintf(context, "lat=1.2343$lng=132.1233");
client.add(VARIABLE_LABEL, value, context);
Ubidots::add(char *variable_label, double value, char *ctext, long unsigned timestamp_val)
Example:
sprintf(timestamp, "1531143251");
client.add(VARIABLE_LABEL, value, NULL, timestamp); // Change for your variable name
I hope this would help you!
All the best,
Maria C.