I have not been able to find the resolution to this issue anywhere. Finally figured it out.
On multiple Android devices I was able to set the manual proxy simply by accessing the advanced network settings and saving the new values. However, on several Androids, the Save button was always grayed out.
All you need to do is populate the Wifi network password field as well. Then the Save button will not be grayed out. So each time you need to update these, first populate the password field, then change the proxy settings, IP, port etc. and it just might resolve your issue.