There are several common ways to contact an administrator. As a rule, such a need arises in case of malfunctions or any additional questions related to the services provided, equipment, etc.
First of all, an administrator is a person who is responsible for the operability of the equipment, the provision of any service (for example, the Internet). In this state of affairs, you have information to contact him in an agreement with the company providing the services. At the very end (or, possibly, the beginning) of the contract, the telephone number for communication with the administrator must be indicated.
If there is no agreement at hand, or the information provided in the agreement is outdated, or there is none at all, you should try to find contacts from friends who use similar services, or find the phone number of the company providing the services, and having already called there, specify the administrator's phone number. If the company is well-known, then, as a rule, there is a contact phone number in the advertisements. If the problem is not very serious, then it is likely that they will be able to help you by the contact phone number, without having to contact the administrator.
If you have access to the Internet at hand, you can try to find the company's website, click on the "contacts" section there, and thereby find an email address for communication. If there is no “contacts” item, then, most likely, the e-mail address for communication is written at the very bottom of the site. It is also possible that the site has a forum, and there is a section for communication regarding the administrator's consultations with users. In extreme cases, using the forum, you can write a private message to the administrator.