I suggest contacting Fidelity again. Fidelity should know whether or not the particular 401(k) will accept a repayment of a CRD. Most will but some will not, governed by the terms of the plan agreement regarding acceptance of rollovers. Such a repayment is to be treated the same as a direct rollover.
If the plan will not accept the rollover, you have the option to instead roll the distribution over to an IRA. You'll need to inform the IRA custodian (which could be Fidelity if you so choose) that the deposit is a repayment of a CRD so that the custodian can report it on Form 5498 with the correct coding in box 13c and repayment information in boxes 14a and b.
