As a Landlord it is your responsibility to ensure your premises are secure. You must provide and maintain locks or security devices as are necessary.
What is ‘reasonably secure’ will vary in different situations.
A Landlord does not have to make the property so secure that it can never be broken into; just provide a reasonable standard of security considering the area of the premises and its potential for prospective thieves.
If rented premises are in a calm and secure neighborhood, security devices and arrangements can be of a minimum. If however, premises are in high risk areas with many incidences of theft, then you should provide adequate security arrangements by installing secure locks on doors and windows, security alarms, etc.
Further security arrangements on lower floor units should be higher than those on upper floors, as lower floors offer easier access to thieves.
Your insurance company requirements are not the test of ‘reasonable security’ but merely another factor to be considered.