Typically, reloading the MTH DCS software cures the problem.
When I first had this happen (25-blinker), this wasn't the case. In response to my issue, first MTH corrected my TIU's problem and then added code to the Loader Program to reset any TIU with an ID# greater than 5, to a DCS ID# of "1" during the TIU software updating process.
From page 175 of The DCS WiFi Companion:
There is one TIU problem that the author has seen on one occasion that has also been reported by a few other DCS operators. In this instance, the TIU’s DCS address becomes outside of the normal range, i.e., between 1 and 5. When this occurs, DCS Remotes will become unable to access the affected TIU. It is unknown, however, if the DCS Application is also unable to access the affected TIU since the author has not experienced this problem while using the DCS Application, and has not received reports from any other DCS Application users in this regard.
This and a whole lot more about DCS WiFi is all in MTH’s “The DCS WiFi Companion 2nd Edition!"
This book is available from many fine OGR advertisers and forum sponsors, or as an eBook or a printed book at MTH's web store!