Couldn't detect external monitor in ubuntu 18.04.5 with NVIDIA driver 455.38

I guess the problem is that the kernel 5.4 that comes with 18.04 is too old to support the AMD gpu. If you really can’t upgrade to 20.04, you’ll have to install a more recent kernel:
https://forums.developer.nvidia.com/t/geforce-gtx-1650-ti-and-linux-mint-19-3/124667/8

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