Help using Linux to recover images from an Android phone

[Steve Mysterious]

Phone:
LG Volt LGLS740
Android 4.4.2

Computer:
Linux Mint 20
Cinnamon 4.6.7

Software:
- Android Debug Bridge (ADB) 1:8.1.0+r23-5ubuntu2
- Android SDK Platform Tools Com 27.0.0+12
- Media Transfer Protocol (MTP) library tools 1.1.17-3
- TWRP - could not find a version of TWRP for my phone

Hello,

I would like to recover some deleted pictures from an old phone.

I didn't have any luck using TestDisk on it directly, so I want to make an
image of the phone and try TestDisk again.

I found these directions on StackOverflow:

1. Download insecure boot.img to your PC from
https://www.androidfilehost.com/?fid=9390169635556426389

2. Reboot your phone into fastboot mode by powering it off and then pressing
and holding volume-down and power buttons.

3. From your Linux PC in the folder where boot.img is located type:
$ fastboot boot boot.img

4. To copy the image of the mmcblk0 partition type:
$ adb pull /dev/block/mmcblk0 mmcblk0.img


I've been stuck on step #2 above.

I haven't been able to get into fastboot mode on my phone.

I mounted my phone:
sudo go-mtpfs -allow-other /media/steve/Steves1TDrive/Phone

I then followed these directions for getting my phone int fastboot mode:
Getting into safe mode:
https://droidrecovery.com/lg-volt-ls740-recovery-mode/


1. First of all, make sure you have installed LG Volt LS740 USB Driver and
ADB and Fastboot Drivers on your Windows PC.

2. Enable "USB Debugging" on your phone from Developer Options.
If Developer Options is missing; go to Settings > About Phone and tap on
Build Number for about 7-10 times.

3. Connect your device to PC and open Command Prompt (CMD).

4. Enter the below command to boot LG Volt LS740 into Recovery Mode.
adb reboot recovery


I skipped the part about "Fastboot Drivers on your Windows PC". Ha ha ha.
All else was okay.

However this command did not work
adb reboot recovery

This is the output I got


$ cd /media/steve/Steves1TDrive/Phone/
$ ls
'Internal storage'
$ adb reboot recovery
* daemon not running; starting now at tcp:5037
* daemon started successfully
error: no devices/emulators found
$


/media/steve/Steves1TDrive/Phone is the mountpoint for my phone.

I realize this isn't entirely a Linux issue, but I would be grateful for any
help getting my phone into fastboot mode so I can complete the instructions I
found for making an image and eventually recovering deleted pictures from it.

Thanks much for any clues

Steve