What is the link between Root canal treatment and Mouth Cancer?

This is a wrong statement. Cancer is not caused by root canal operations. This myth stems from false statements that 97 percent of persons with terminal cancer had root canal surgery, implying a relationship between dental operations and cancer development.
Since the 1920s, there has been a widespread belief that root canals are a major source of cancer and other serious disorders. This misconception is now widely disseminated on the internet. It arose from the work of Weston Price, a dentist in the early twentieth century who conducted a series of inaccurate and poorly conceived tests.
Based on his personal studies, Price concluded that dead teeth that had undergone root canal therapy still harbored extremely dangerous poisons. Toxins, he claims, serve as a breeding environment for cancer, arthritis, heart disease, and other diseases.

What exactly are root canals?

A root canal is a type of dental crown procedure that is used to heal damaged or infected teeth. Instead of extracting the affected tooth, endodontists clean and fill the canals by drilling into the core of the tooth's root. Endodontics is the branch of dentistry that investigates and treats illnesses of the pulp of the tooth.

A tooth's interior is loaded with blood arteries, connective tissue, and nerve endings, which keep it alive. This is referred to as root pulp. A crack or hollow in the root pulp can induce infection. These microorganisms can cause complications if left unchecked. 

These are some examples that include tooth abscess, bone loss, swelling, and infection or symptoms of a dental abscess. If the root pulp becomes infected, it must be treated as quickly as possible. Root canal therapy or extractions are the two basic therapies for infections of the root pulp.

Root canals and fear of Cancer

People who receive root canal therapy are no more or less likely than anyone else to become unwell. There is almost little evidence to suggest a link between root canal treatment and other disorders. Contrary rumors can generate a lot of stress for a lot of people, including former and upcoming root canal patients.  
Some people who have had root canals will even have their dead teeth pulled. They see this as a precautionary measure because they feel a dead tooth raises their chance of cancer. Pulling out dead teeth, on the other hand, is needless. It is always an option, but dentists believe that saving your original teeth is the best alternative.

Then what can cause mouth cancer?

The reasons for the cause of mouth cancer can be numerous including usage of tobacco in the form of cigarettes, cigars, pipes, chewing and snuffing, chewing betel nuts, heavy use of alcohol, poor diet. Another important reason can be the use of improper dental crowns or implants. There are certain types of materials that are known to cause oral cancers, particularly dental crowns made in China.  
A tooth crown in Chinese or a dental crown in Chinese, called Yá guān is known to be a mage of material that contains radioactive substances. These crowns are made available at cheaper prices but are life-threatening.
The materials like zirconia must be used in order to get a more natural finish with other features like bio-compatibility, strength, durability, and risk-free use. Some companies like 3MM & Dental Direkt offer Zirconia products with high quality and comparatively low prices.