Stuck installing onto a new to me Uquiti ER-X

It had 2.0.4 earlier this evening which is what it had when I bought it. suggested going down to 1.10.10.

I have factory files from here

In both 2.0.4 and 1.10.10 ssh or UI I get told the image is not acceptable.

Checking upgrade image...Upgrade image does not support the device. Upgrade failed. says the same and that serial worked.

I do not currently have a serial adapter. I haven't used one since the db9 days...... what should I buy? Or is there are a way that works without cracking the case?

The inbox in the URL means this is a work in progress article. This is the official page:

CP2102 works well, I would recommend cracking the case as it's really easy to take apart and the serial is prepopulated see:

I spotted the router to have just two small screws and decided to remove these. After this, the casing opens right up without breaking any stickers and as such this shouldn't affect your warranty.