How To Trick WeChat Into to Thinking My iPhone Is An iPad?
The WeChat app can ask iOS whether it is running on an iPad or iPhone. It is not something you can set in Settings, or "fake" through the Messages settings as indicated elsewhere.

The only way to overcome this is with a jailbroken iPhone, where you can intercept the relevant method calls. In practice it is not possible to do so for an "ordinary user" without strong technical skills.

Try contacting WeChat support to see if they have a work-around, or they might consider lifting the restriction in coming versions.