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I did that and found it but I got this message:
Product delisted, Reason: this product is obsolete.

Is there any other way to install this app?

See if it lets you install Mywi 5.0 or if the latest MyWi 8.0 will work on ios 5.
I'd contact the Dev of MyWi to see if he will let you purchase the old version and puts it up on a repo somewhere and Im sure he will be able to give you more details.
 
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