I flashed just now and came across the same problem. Used openwrt-ramips-mt7621-dlink_dap-x1860-a1-squashfs-factory.bin from https://github.com/RolandoMagico/openwrt/releases/tag/Build_2022-12-22, which i renamed to "factory.bin". Followed these instructions: OpenWrt Support for D-Link DAP-X1860 - #69 by dKoco, except that I first tried to use it with Firefox. When I did so, i encountered the 1259 Bytes problem (the file should be way bigger!). Then I tried the same with Microsoft Edge and it showed a file size that I deemed more accurate (but I did not crosscheck), so I flashed and it worked just like described in instructions. I am not completely sure if using Microsoft Edge fixed it, or if it was a timing issue.
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