I think your mixer is set up to have main outputs on 31 & 32, but I don't know how REAPER will default -prob outs 1 & 2 as suggested above. Do you know how it is sent?
Everything JP has said seems to be correct for what you're trying to do -mix ITB. For outboard use in mixing you'll have to send to outputs and return on inputs that are set to straddle a console channel's insert and EQ (option-8, A&B selected, page 39). Rendering is then done online in real-time to allow real-time signals to pass through your outboard FX.
Get familiar with that desk's routing and your gain-staging first and mix ITB. You can try the more ambitious tasks when you're more confident -you've got a lot to absorb Sean
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