Oh, bother, and I missed that the chip on the RS485 shield does not have automatic flow control and the DE and REnot aren’t even on the same pin. The library has support for a single DE/REnot pin, but for two separate pins. You’ll need to physically join those pins for the library to work. Run a jumper wire between A4 and A5 and then use either one of those for the DEREPin for the library. In the past I’ve had almost no success in getting manual direction control to work properly.