So, I ordered one of those clamps (1.27mm, double row 10pin) https://aliexpress.ru/item/1005001409579446.html?gatewayAdapt=glo2rus&sku_id=12000021128795872
(about 3 weeks shipping 
)
I have a chance to buy used raspberry pi 4 in my place not particulary for this purpose, but it seems that it's possible to use this board as JTAG debugger:
Is this a viable option for this router or RPi GPIO interface is no go for this kind of mission and I need that FT2232-like board?
And if so my next steps will be just flashing recovery image via JTAG or I can somehow (I am a newbie in this JTAG\UART thing 
, just connecting pins here and there avoiding 5V pins 
) examine a source of issues from my first reflash attempt?