A truly void judgment can always be set aside; you need to first file in the trial court a motion to vacate the judgment; if that is denied, you can appeal from that order. Whether you can now appeal directly from the judgment depends on when the judgment was entered and other factors.
That being said, a truly void judgment is a rarity. You have to convince the judge that it was procured by "extrinsic" fraud rather than "intrinsic" fraud, the definitions of which are too complicated to explain here. As you provide no specific details of the "fraud" (I do not know what you mean by "procuring subject matter jurisdiction") it is impossible to provide further advice.
You should, of course, retain counsel to assist you in this.
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You should speak with an attorney and have your "evidence" supporting documents etc in hand. This is not a simple proposition and not something a lay person should undertake without the assistance of counsel
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