Checked technical docs, WEP keys can be the following or more:

40- or 64-bit WEP: 10 digit key (5 ASCII characters)

104- or 128-bit WEP: 26 digit key (13 ASCII characters)

256-bit WEP: 58 digit key (29 ASCII characters)

Unfortunately in our UI we only deal with 5 or 13 ASCII characters. OpenWrt can deal all of this. So we have to improve the UI.

You are correct about 2.4G/5G.