Sorry for the late reply. Did you get the phone to supply power via its USB port with postmarketOS? I did not get that to work. According to the wiki, this is a known issue: “The phone won’t supply its own power, so you need either a powered hub or an extension cable wired up to 5v. Some hubs may refuse to work without power being supplied from the port however.” See the OTG doesn’t work on the Wiki page

Are you using external power with the Adapter? I ran into this issue with my oneplus 6t, which cant supply power to a usb device. Using a external chsrger with a Combo dongle works fine right now. You may need to enable host mode though