Best posts made by papadeltasierra

@teastain2 Upload, absolutely agree. I did have a very "left field" thought this morning. I wonder if the tlsr8258 boots up in "USB mode" and plugging in the T-U2T makes it negotiate to Serial mode which then makes things work.
In the repo that you referenced, the tlsr8258 firmware is just "a binary blob" so I cannot see what it does but I will take a long look at my tlsr8258 code this evening. I know there are options to enable to tlsr8258 library's ZBHCI interface (the way you control the ZigBee operations of the board) so maybe I've set one of those wrongly.

@papadeltasierra Aha, there is indeed 4 MB of flash! You need to use idf.py menuconfig and then set the Serial flasher config to 1Flash size 4 MB and then use Partition table to select a 4 MB usable partition table.