How can I increase the number of iterations per epoch in MATLAB?
I think you misunderstand the difference between the terms:

Epoch - A single pass over the entire training dataset.

Mini-batch - Number of samples to pass thru the model before performing a single update of its weights.

Iterations - Number of required mini-batches to complete a full epoch.

Example:
you have a training dataset with 256 samples, a mini-batch size of 32 and epoch number 100.
Each training sample will be viewed by the model 100 times (as per the epoch number).
Each epoch will be done in 8 iterations (i.e. the model will be updated 8 times in an epoch).
Each iteration will include 32 training samples passed thru the model. The back propagation algorithm will calculate the required change in the model weights for every single sample, but the actual update will take place on the mean (or sum) of all 32 samples in the mini-batch.

As you said at the end, "iterations per epoch are set automatically according to how many training examples there are present".
There is no reason to alter this parameter, if you want to change the number of passes over the data, use the epoch number.