In a personal injury action, is defendant permitted to appeal his motion to vacate judgment for lack of jurisdiction?

In 2016, defendant filed a self-styled "Motion To Vacate Judgment For Lack of Jurisdiction" raising a single claimnamely, that the trial court lacked jurisdiction because the original complaint filed against him was signed by someone other than the complainant in violation of section 806. The trial court summarily denied the motion, and defendant filed a timely notice of appeal. His appointed counsel filed a brief pursuant to People v. Wende (1979) 25 Cal.3d 436, raising no issues. We invited defendant to file a supplemental brief. Defendant filed a letter brief renewing his challenge to the court's jurisdiction, and raised two entirely new arguments. We sought additional briefing from the parties on one of the two new arguments defendant raised in his letter brief.