I don’t know for sure how LMIC determines the first data rate to use for OTAA. (Indeed, I thought it started joining using whatever was set using
LMIC_setDrTxpow, but maybe not.)
However, an OTAA Join Accept downlink is not much different from a regular downlink (except for RX1 being 5 seconds rather than 1, RX2 being 6 seconds instead of 2, and for RX2 in EU868 using SF9 like configured automatically in the EU868 Join Accept). So, it may be easier to test regular downlinks first (maybe even using ABP, along with manually setting
LMIC.dn2Dr = DR_SF9 for EU868).
Also, to increase the chances a specific gateway is used (if you suspect that some gateway may not transmit the Join Accept, like due to network latency of that very gateway), you may want to move closer to a specific gateway, to see if that helps. If duty cycle limitations allow for it then TTN will usually select the gateway with the best reception of the uplink (Join Request) to transmit the downlink (Join Accept).