Hi,
Well, to provide a solution, I would first need to reproduce the problem and then understand what is the problem.
I was able to reproduce the issue with the default browser of android 4 and 6 using the emulator genymotion.
From what I can see, the issue is that there is some javascript error on the page (which could be due to HikaShop's javascript or another extension's javascript on the page), leading to the javascript on the buttons of HikaShop not working.
Because of that, when you click on the "add to cart" button, it will directly redirect to the checkout, instead of displaying the notification box, like on a recent android device. And also because of that the "finish" button of the checkout doesn't work.
Now, we would need to know the javascript error to be able to move forward.
Looking online, I can see that it's possible like this:
android.stackexchange.com/questions/5999...do-those-settings-do
However, I wasn't able to activate the debug on my emulator and I don't know why.
Are you able to do it on your devices ? If so, what do you get in the javascript console ?