It sounds like you have a good understanding of the basic methodologies and paradigms that can be used in research. You mentioned mixed methodology, which is an approach that combines both quantitative and qualitative research methods to provide a broader understanding of the research topic. This can be a good approach for your research topic, as it allows you to gather both statistical data and more nuanced insights into the reasons behind poor performance in mathematics.

In terms of paradigms, you will need to consider whether you want to adopt a positivist or constructivist approach. Positivism emphasizes the use of objective, quantitative methods to study social phenomena, while constructivism emphasizes the subjective and contextual nature of human experience. Both paradigms have their merits and drawbacks, and the one you choose will depend on your research question, methods, and objectives.

Regarding epistemology and ontology, you need to consider what is the nature of knowledge that you are trying to acquire and what is the nature of reality as you perceive it. Epistemology deals with issues of knowledge acquisition, and ontology is concerned with the nature of reality. You will need to consider these issues in depth to determine how they relate to your research topic and how you can address them in your research design.

I think a mixed-methods approach is a good choice for your research topic, but you will need to carefully consider your paradigm and epistemological and ontological assumptions to ensure that your methods and techniques are well-suited to your research question.

Going with the research topic as mentioned in your question and the overall concern related to your research. The topic can be studied from three perspective, one from the students perspective, second from the perspective of teachers who teach mathematics and the third from the perspective of the subject itself. These three perspective can help you in building your research paradigm. When you work on the the research paradigm based on aforesaid three perspective you shall be able to figure out your own theoretical framework or if not the theoretical framework even then you probably would be knowing what exist related to poor performance in mathematics. The details of what already exist is the explanation of your ontological aspect. For example, you may got to know some common problem of students with mathematics or even what aspects of mathematics positioned it as a complex subject. All this would be part of ontological aspect. Now coming to epistemology, When as a researcher you try to explore how people know about what is their problem then you are conducting epistemology. For example, when you ask question as an attempt to figure out what made students to realize their issues or concerns with math then this is the part of epistemology. Now here is a catch! as a researcher while conducting research the boundaries of ontology and epistemology got blurred because for example when you ask the respondent, what is you most common problem with the subject math, followed by next question how do you know this. The first question is about ontology while next question is about epistemology. Positivism and constructivism are two ways to verify knowledge. If you decided to use some scientific method like experiments or algorithmic proof then you are following positivism and of you use a method that explore the interaction and level of intelligence from a subjective view point then your methods are more close to constructivism. In your case my suggestion is to adopt constructivism.

You may read the following for more details:

https://www.nicole-brown.co.uk/epistemology/

https://jacknewmanhe.wordpress.com/2020/04/27/simplifying-some-isms-and-ologies/

Hope this will be helpful

Is there some reason why you think you need a paradigm, since that is a relatively old-fashioned approach to research. In particular, mixed methods research is more likely to emphasize the match between meaningful research questions and appropriate methods for addressing those questions -- rather than abstract issues of ontology and epistemology.