This is just the process I did..
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Install firmware-atheros bluetooth bluemon bluez* rfkill rfcomm dhcpd
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Update Kernel to 5.7 using dpkg it will generate new initramfs (Don't reboot yet)
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Blacklist ath10k_pci module
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copy rampatch_00440302.bin and nvm_00440302.bin to /lib/firmware/qca/ (the required files may be different if you have a different variant 950XL, check dmesg for what firmware fails to load)
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Add entry to grub.cfg (Example of mine):
`set timeout=07
menuentry "Debian Kernel 4.18.6" { set background_color=black insmod part_msdos insmod ext2
set root=(hd0,gpt41) linux /boot/vmlinuz-4.18.6-lumia-950xl-uefi root=UUID="969daa87-f176-4283-9d80-bbd899a687a8" video=efifb acpi=no rw initrd /boot/initrd.img-4.18.6-lumia-950xl-uefi
devicetree /boot/msm8994-microsoft-lumia-950xl.dtb
} menuentry "Debian Kernel 5.7" { set background_color=black insmod part_msdos insmod ext2
set root=(hd0,gpt41) linux /boot/vmlinuz-5.7.0-lumia-950xl-uefi root=UUID="969daa87-f176-4283-9d80-bbd899a687a8" video=efifb acpi=no rw initrd /boot/initrd.img-5.7.0-lumia-950xl-uefi
devicetree /boot/msm8994-microsoft-lumia-950xl.dtb
}`
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Reboot
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Make sure modules and firmware are loaded (btusb bnap ath10k_core and the .bin files with dmesg)
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Done, you should now be able to connect to Bluetooth networks (Pan/Dun etc)