How to fix a brick planter attached to my house

Can you post a photo?

Why is your brick crumbling? Was it painted at some point? That traps moisture which then damages the brick when it freezes each winter. If the brick was never painted then it may just be inferior quality brick. It is almost impossible to match new brick to old. Even if you buy from the same manufacturer, it will look different. I think you need to have a mason look at the problem. You may just need to remove crumbling mortar and replace it with new mortar. Mortar can be tinted, so the mason may be able to make the new mortar look similar to the old.

If the bricks are spalling (surface peeling away in flakes) or crumbling, I don't know if you can repair them, but the second link below has some links to other articles on that. Most damage to brick is caused by a concentration of moisture. Does water drip from the roof into the planter? Is there a gutter that overflows or a downspout that empties too near? Look for such a situation and try to remedy it.

The same moisture damage happens to mortar. But if the mortar is cracking or coming loose in the joints, you can fix that yourself. You will needs some tools, but the job can be done in sections. In fact it's best to mix the mortar in small batches as it starts to set up as soon as it is mixed. Here are some links:

https://www.thisoldhouse.com/how-to/how-to-repair-mortar-brick-wall

https://homeguides.sfgate.com/repair-deteriorating-brick-wall-27350.html

https://www.dummies.com/home-garden/exteriors/brick-repair/how-to-patch-mortar-on-brick-walls/

One thing you should always do is wet down the brick before you start applying mortar. If the brick wall is dry it will suck the water from the mortar and cause it to dry too quickly. You should cover the area of new mortar with a wet cloth such as a canvas tarp or an old, worn bedspread, especially in hot or dry weather. It takes about 3 days for mortar to cure, then brush any excess from the bricks.