Okay so I tested on my side and I got it working with the firewall. You need to insert the rule in the private-subnets section before the last rule that block everything.
Code
13 ACCEPT all -- 192.168.0.0/16 anywhere ctstate NEW
14 ACCEPT all -- 10.0.0.0/8 anywhere ctstate NEW
15 ACCEPT all -- 172.16.0.0/12 anywhere ctstate NEW
16 ACCEPT all -- 192.168.0.0/16 anywhere ctstate NEW
17 ACCEPT all -- freeplayer.freebox.fr anywhere
18 REJECT all -- anywhere anywhere reject-with icmp-port-unreachableYou can achieve this with this command :
iptables -I private-subnets 17 -s mafreebox.freebox.fr -j ACCEPT
Then to save the rule after the next reboots, type this command :
iptables-save > /storage/.config/iptables/rules.v4
Finally, reboot your device and change the firewall config from Home to Custom and you should be good to go.