I've treated a lot of foster dog ear infections. Mometemax is great (and there are other brands that are similar ingredients, RX only). The big downside is that you MUST clean the ears before applying it, for it to work as intended -- and dogs with ear infections hate having their ears cleaned. You also need to re-check in a week to make sure it's responded appropriately, as resistant microbes are now on the rise.
Zymox doesn't have the resistance problems because it uses an enzyme to break them down, but it's also not as strong as the RX. OTOH, you do not clean the ears before applying it, so it's brilliant for dogs who hate ear cleaning (or for whom it's painful).
The Zymox product line can be a little confusing. The "ear treatment" products are labeled as "otic enzymatic solution," for dogs with problems (like yours). They offer them with and without hydrocortisone (which helps the itching), and in different strengths. For dogs with chronic issues, I would use the red bottle ("advanced") -- there's a new gold bottle, but I haven't used it and am not sure the difference from the red bottle. For a one-off, the regular strength blue bottle (regular strength) is fine -- especially if you're good at starting it as soon as you notice the dog pawing at the ear and don't let it progress. Chewy has them all:
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After you have treated it, then you may need some maintenance cleaning. The Zymox Cleanser is the best ear cleaner that I know of. I keep a big bottle around.