I'm no expert on DCS, TMack, and hopefully some such experts will give you the advice you need, but I got a DCS remote unit and TIU about 3 years ago, got Barry Boscowitz' Bible on DCS, worked hard for 3 weeks during Covid to re-wire my layout and install DCS using the star wiring system, and my DCS has worked like a charm ever since.
I'm not sure, but since it sounds like your DCS remote and TIU are old, you might have an old version of the TIU, which I believe can be upgraded to the L version, which is the latest version that is much better than the old version. and that can be done relatively economically. Again, hopefully someone more knowledgeable will chime in soon about this.
By the way, the issue of the various versions of the TIU was discussed on another recent thread, so I suggest you do a Forum search for it.
I love the DCS remote unit/TIU system, and from what I understand, it's no longer made, so if your system can be upgraded and installed properly, I think it is very worth your while to do so.
I only use the basic functions of my DCS remote and TIU, which includes adding PS2 and PS3 engines, read engine, start engine, shut down engine, speed control, horn, bell, and electrocoupler, and I only run one MTH PS2 or 3 locomotive on DCS at a time. I could be wrong, but I think that limiting my DCS use to these basic functions helps keep my DCS system trouble free.
Also, although I have the DCS App on my smartphone, I don't use it; I only run locomotives on DCS using my remote unit.
One more thing: I know nothing about the DCS WIFI system.
Hope this helps, and hope you get others more knowledgeable than me to chime in. Arnold