OK, so I gues I'm able to upload right firmware via Serial (using Kermit 95 on Windows 10...)
I just have no idea about the right adresses. If I will upload sysupgrade file, are those the right parameters please ?
deleted, possibly wrong
As mentioned https://forum.archive.openwrt.org/viewtopic.php?id=52625&p=8 for C2 AC750 ? Can I simply download sysupgrade from http://downloads.openwrt.org/releases/19.07.0/targets/ramips/mt7620/openwrt-19.07.0-ramips-mt7620-ArcherMR200-squashfs-sysupgrade.bin and follow this ?
...update...
Following https://git.openwrt.org/?p=openwrt/openwrt.git;a=blobdiff;f=tools/firmware-utils/src/mktplinkfw2.c;h=3ab5c52ec2a3b26f927fe76f63e4a0fc67f35b45;hp=213e6729a48877eaecfbaf8de760b44bf03c699e;hb=c8043137bbd323b1490ae8613eab915ba9c138ee;hpb=190ee7d86b450083fea4236d588d0d88a50e1311 and https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/ramips/dts/ArcherMR200.dts;h=93fb048d49b14564233a465a426e68334b939991;hb=c8043137bbd323b1490ae8613eab915ba9c138ee I'm not sure the values above are correct.
122
123 partition@0 {
124 label = "u-boot";
125 reg = <0x0 0x20000>;
126 read-only;
127 };
128
129 partition@20000 {
130 label = "firmware";
131 reg = <0x20000 0x7b0000>;
While sysupgrade file don't have bootloader included, what are the right values please ? The size is probably 0x7b0000, initial adress "after" bootloader is 0x20000 and the address where to load the copied firmware ? I found somewhere else 0x80060000.
loadb 0x80060000
erase tplink 0x20000 0x7b0000
cp.b 0x80060000 0x20000 0x7b0000
bootm
Last update : so update via serial (KERMIT 95 ;-)) ) and sending sysupgrade file above with following settings in the KERMIT was SUCCESFULL.
SET MODEM TYPE DIRECT
SET SPEED 115200
SET CARRIER-WATCH OFF
SET FLOW NONE
SET PARITY NONE
SET CONTROL PREFIX ALL
SEND ROM_SYSUPGRADE.BIN
Somehow KERMIT didn't allow to interrupt booting process using "t" key, so I had to use Putty, connected to Terminal, interupting via "t" key, then closing, connecting again via KERMIT - sending the file and then - finalising via erase, cp etc. Kind of tricky yet was working.
NO IDEA while UBOOT was without Ethernet but it's good to know there is yet KERMIT 
Btw, it was possible to download only non-crypt version of Kermit but it was ok... (http://www.columbia.edu/kermit/ftp/trial/k95_21_nocrypto_vbox.exe).
Hope someone will find this useful in case of brick.
Last note - I used ttl USB adapter connecting only GND, RX and TX (RX and TX switched). During the operations I got garbled output - then I found out it was not soldered correctly.
KERMIT SERIAL PORT setttings, terminal VT100 :
