Try to add that client to access list with 'Allow Signal Out Of Range' parameter set.

eg. GPON_SN ABCD12345678
You must take serial number from provider (Fritzbox) or make it by yourself. I have Huawei ONT and to have SN must take first half of the number and convert it trough HEX -> ASCII, eg. Huawei: 48575443 -> HWTC and the second half you must use instead of 12345678

My situation is similar but i solved it like that: I have gree AC (4 appliances) they do not want to connect only to the caps with only ax network. Using my provisovision section I have a hybrid structure of 2.4 IoT network: main 2.4g network with N and slave working in AX standard. My gree devices ...