I don’t think the issue is on the routers side. I would take a look on what the SSH client is sending.

On a Linux SSH client, start with the setting echo $TERM
Depending on the used terminal emulator (xterm, terminal, term, …) There are different ways of ‘correct’ the command, send by keypress.

For example, edit the file .Xdefaults with:

XTerm*VT100.Translations:
 <Key>F1: string(0x1b) string("[A")
 <Key>F2: string(0x1b) string("[B")

And execute:
xrdb -merge <~/.Xdefaults

Bit this is a very specific solution, without knowing your setup. It may not work.

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