How to custom Resolutions in M1 PRO 13 inch

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Hi all. I using Macbook m1 and using screen DELL P2421D. I have problem when i connect from macbook to extends screen. When i connect to DELL P2421D, my font screen very fuzzy. When i check my screen, it not enable HiDPI. I try download RDM new version 2.5.0. My screen max resolution is 2560 * 1440. I custom it enable HiDPI nothing happy. It only show me no option hiDPI screen. So when i choise option Display mirroring on RDM. Well now it enable HiDPI on my screen P2421D and macbook. My screen another enable 1680:1050. Now font P2421D verry good. But now it keep content my screen macbook and P2421D . How to only choise option HiDPI on DELL P2421D . I try set some option of screen macbook support hiDPI but nothing happen. I try using ResX and when i open monitor it show me 1680:1050 enable hiDPI . But when i cancel mirroring and custom 1680:1050 in P2421D it not working. I don't know why i can't enable it. Please help me enable custom HiDPI with 2560 * 1440 or less than. Because when i using 1680:1050 it small . Please help me thanks you