Hi,
Only the models that belong to the same sub-series can be used to set up an istack. That is, S5731-S can only set up a stack with S5731-S, the same applies to S5720-LI.
You can use the Stack & SVF Assistant tool to query and solve any stack-related problems.
https://support.huawei.com/onlinetoolsweb/virtual/index?lang=en