Scientifically, we'd like to mount you with pressure-sensors, especially ulnar and thenar pads. Numbness is generally point-pressure blocking blood flow to nerves. What's going numb?
Index and second finger, ring and pinkie?

As Botto said, increase your core strength.

Changing positions should help: halfway on the hoods, all the way on them, tops, halfway on the drops, way down on the drops. If you had to, aerobars spaced widely enough to give you comfortable stability, no hand pressure at all.

Changing stem length and angle, bar type, saddle position, to help you get weight relief while strengthening your core muscles.

You can also try thick Barphat tape and gel pads (which may be on your Allez, stock on Roubaix). If it works, it doesn't mean you have to use it forever.

When you're starting out, what you need to feel comfy with isn't the same as after you've been riding longer.

Learning to "ride light and loose" on your bars takes time. Numbies happen. They're fixable.