Don't you see the difference between what should work and what works for me?
Missing (IPQ) # prompt, and can't enter the commands
This doesn't look like a boot log with serial connexion.
Treat the router as being bricked (probably it is) and recover with the TFTP and serial connection method. This OpenWrt EA8300 webpage has excellent information. Look into the section "U-Boot" and setup a TFTP server according to the IP addresses there. Change the name of the router firmware (OpenWrt, Linksys stock or even DD-Wrt) to "dallas.img". If you flash with OpenWrt firmware, you have to use the factory.bin, not sysupgrade.bin. Also for EA8300 firmware beginning with 23.05.0) you have to change the u-boot variable kernsize from 300000 to 500000 first. The linked webpage above has detailed information. You can also go back to Linksys' stock firmware if you want to (just change the file name). I used the method to flash EA8300, MR8300 and experimental MR9000 firmware into an MR6350 when I tried to get MR6350 working (with OpenWrt) a few years ago.
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@git72
Flash Linksys in order to restore the router to its original state, than flash it again to have both partitions installed. You will be sure of the initial state
In that case better flash 22.03 first, to start from a fresh and stable system.
Change the boot variable than Install 23.05 later.
I have tried everything step by step. The only thing I have done PREVIOUSLY is that I changed the u-boot variable kernsize from 300000 to 500000.
Is it possible that this is where the error is coming from?
Yes, it is a safer approach. When I did this recovery method, it was 22.03 era.
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Do as @badulesia suggested - go back to the Linksys stock firmware and start from there. Also research if Linksys has its own recovery method for bricked device.
Apparently my English is very bad. You don't understand: it DOESN'T happen - neither to go back to stock, nor directly to OpenWRT.
I put a pic of the END of the serial port connection - it doesn't get to where I can write a command !!!
Using a serial link, can you post here a serial log of the boot process?
What you posted before doesn't look like a serial log of booting.
For example, this is the start of the boot process on the MR8300, a device close to yours. I ended it where the boot allow to interrup, and enter u-boot console. You should have something very similar.
You must hit a key to interrup booting. While in u-boot console you enter some commands to setup a TFTP connexion. Use a PC as TFTP server.
https://openwrt.org/toh/linksys/ea8300#u-boot
Format: Log Type - Time(microsec) - Message - Optional Info
Log Type: B - Since Boot(Power On Reset), D - Delta, S - Statistic
S - QC_IMAGE_VERSION_STRING=BOOT.BF.3.1.1-00108
S - IMAGE_VARIANT_STRING=DAACANAZA
S - OEM_IMAGE_VERSION_STRING=CRM
S - Boot Config, 0x00000025
S - Reset status Config, 0x00000000
S - Core 0 Frequency, 0 MHz
B - 262 - PBL, Start
B - 1341 - bootable_media_detect_entry, Start
B - 2618 - bootable_media_detect_success, Start
B - 2632 - elf_loader_entry, Start
B - 4048 - auth_hash_seg_entry, Start
B - 6215 - auth_hash_seg_exit, Start
B - 71421 - elf_segs_hash_verify_entry, Start
B - 193960 - PBL, End
B - 193984 - SBL1, Start
B - 286143 - pm_device_init, Start
D - 7 - pm_device_init, Delta
B - 287642 - boot_flash_init, Start
D - 87496 - boot_flash_init, Delta
B - 379449 - boot_config_data_table_init, Start
D - 14033 - boot_config_data_table_init, Delta - (419 Bytes)
B - 396176 - clock_init, Start
D - 7575 - clock_init, Delta
B - 407190 - CDT version:2,Platform ID:8,Major ID:1,Minor ID:0,Subtype:6
B - 410603 - sbl1_ddr_set_params, Start
B - 415697 - cpr_init, Start
D - 2 - cpr_init, Delta
B - 420080 - Pre_DDR_clock_init, Start
D - 4 - Pre_DDR_clock_init, Delta
D - 13172 - sbl1_ddr_set_params, Delta
B - 433385 - pm_driver_init, Start
D - 2 - pm_driver_init, Delta
B - 504079 - sbl1_wait_for_ddr_training, Start
D - 28 - sbl1_wait_for_ddr_training, Delta
B - 520174 - Image Load, Start
D - 143955 - QSEE Image Loaded, Delta - (269176 Bytes)
B - 664557 - Image Load, Start
D - 2120 - SEC Image Loaded, Delta - (2048 Bytes)
B - 674736 - Image Load, Start
D - 187303 - APPSBL Image Loaded, Delta - (444135 Bytes)
B - 862435 - QSEE Execution, Start
D - 56 - QSEE Execution, Delta
B - 868658 - SBL1, End
D - 676676 - SBL1, Delta
S - Flash Throughput, 2086 KB/s (715778 Bytes, 343133 us)
S - DDR Frequency, 672 MHz
U-Boot 2012.07 [Chaos Calmer 15.05.1,r35193] (Sep 27 2018 - 09:37:31)
CBT U-Boot ver: 1.3.3
smem ram ptable found: ver: 1 len: 3
DRAM: 512 MiB
machid : 0x8010006
NAND: ID = 9590dac2
Vendor = c2
Device = da
ONFI device found
SF NAND unsupported id:ff:ff:ff:ffSF: Unsupported manufacturer ff
ipq_spi: SPI Flash not found (bus/cs/speed/mode) = (0/0/48000000/0)
256 MiB
MMC: qca_mmc: 0
PCI0 Link Intialized
In: serial
Out: serial
Err: serial
machid: 8010006
flash_type: 2
Net: MAC0 addr:0:3:7f:ba:db:ad
PHY ID1: 0x4d
PHY ID2: 0xd0b1
ipq40xx_ess_sw_init done
eth0
Updating boot_count ... done
Hit any key to stop autoboot: 0
If it's not recognizing that you hit a key, it could be the transmit wire from your serial adapter to the router is not connected or connected to the wrong place.
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Yes, it had happened to the best to make an pin error with this. Other classical case is inverting RX and TX. Here is EA8300 pin scheme.
https://openwrt.org/toh/linksys/ea8300#serial
and also this with a picture
https://openwrt.org/toh/linksys/mr8300#serial
Okay, now it's starting to make sense...
With this connection scheme, after switching on, nothing happens. That's why I tried the scheme for ea8500 (there TX and RX are swapped) and the serial port responds, but as you say - the stop button - does not respond...
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Thank YOU for dealing with my problem. I'll have to treat YOU 
Thank YOU so much!!!
It worked!
I now have a stock firmware on one partition and OpenWRT on the other!
I'm stopping for today because I got overexcited...
I'll keep YOU updated in the future!!! 
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...I forgot to tell you that in fact, indeed the serial port is like on the EA8500 - 0,RX,TX,0,Gnd , and was obtained after much key spamming ! 
Congrats ! It was worthy to persevere.
And ... no more DD-Wrt "experiments" 
Hello again!
First the report:
I installed the latest stable raelization (OpenWrt 23.05.5 (r24106-10cc5fcd00)), but was surprised that it installed on both partitions of the device - I was hoping that the alternate partition would remain Linksys' stock partition, but apparently the new OpenWRT is being forced and self-installing.... health be it - that's fine with me.
However, there is one problem that I will ask those in the know to help with:
After I set up the ea8300 and everything goes, I also tested the speed: - the download speed is around 200mb/s, and the upload speed around 400.... With the pre-hid router (huawei ax3000) the speeds were 400/400 !
What could be the reason ?
Hi.
On a MR8300, I'm using OpenWrt on both partitions for a couple of years. Nothing to complain about. Just think to install luci-app-advanced-reboot to go from one to another. You can still flash Linksys GUI is you want, but honestly it's not worthy.
Since 23.05, OpenWrt uses DSA that has an impact on performances. One may consider remaining on 22.03. this has been debated on another thread. That being said 200 mb/s is rather slow, I have much faster. Something else is at stake (ISP, switch ?)
Just now, I tried another router (haier_har-20s2u1), which is basically a bridge (second router) with the same OpenWrt 23.05 and the same settings - it works normally 400/400 ????
... well, I continue with the experiments:
I installed on the alternate partition Linksys stock firmware; I started it and it turned out that the speed is fine there too - 400/400.
Now can you tell me how to reboot the other partition (from the stock one)? 