You may be aware that an NRPN message is made up of a series of 3 or 4 Control Change messages.
The official MIDI specification is here: https://www.midi.org/specifications
Here is an unofficial tutorial: MIDI NRPN and RPN by Phil Rees
So an NRPN message is made of two initial CC messages (99 and 98) to say which parameter is to be adjusted, and then one or two CC messages (6 and maybe also 38) to give the value of the parameter.
You need to look at the manual for the DSI synth to determine which values of these messages need to be sent.
You didn’t specify which synth, so here is an example for the Prophet '08:
Layer B LFO speed is NRPN number 237; the value of this NRPN to get “sequence speed divided by 1.5” is 158.
Therefore the four CC messages and values to send are:
CC 99 (value 1)
CC 98 (value 109) [because 1 x 128 + 109 =237]
CC 6 (value 1)
CC 38 (value 30) [because 1 x 128 + 30 = 158]
So you would need to send these four CC messages with these values in close succession from the OT.