If your solar panel has a regulator, it would be competing with the mppt built into the Redarc. Bypass the panel's regulator when connecting through a dcdc charger (I left the regulator in place and rewired to bypass).
Of course, if it hasn't, you will kill your battery by using an unregulated power supply connected directly.
Edit..A Redarc bcdc out of the box, has no terminations on wiring. Any Anderson plug was added by your sparkie and I'm assuming done correctly.