DIY WiFi: coectivity issues with CyanogenMod 12.1

I have a home network built around an ancient computer that I've turned into a router, ruing Debian Wheezy. The wireless part is based on a TP-LINK TL-WN851ND PCI card on the hardware side, and uses hostapd on the software side.

Now, I have recently obtained a new phone, on which I am ruing CyanogenMod 12.1 (equivalent to Android 5.1). This device can coect to my wireless network, but most network operations (e.g. downloading things) result in timeouts, requiring retries. Fetching emails over IMAP is particularly terrible, almost invariably resulting in timeout exceptions. If a coection succeeds, it's fast, but getting the coection to succeed is problematic. It is only on this network that I'm getting these problems. Other devices, in particular a laptop ruing Gentoo Linux and an old phone ruing stock Android 4.x, have no problems on this network. I had a phone ruing CyanogenMod 11 (now broken) that occasionally had trouble coecting but usually worked, but the new one usually doesn't. This leads me to believe the problem is with my wireless network rather than with the phone.

One thing I have discovered just now is that pinging the gateway and any wired hosts within the LAN using the phone always gives 0% packet loss, whereas pinging external hosts or bridged VPN hosts averages about 50% packet loss. Pinging the same external hosts from wired computers in the LAN does give 0% packet loss. I've used bare IP addresses to test this, so DNS isn't the problem.

hostapd configuration:

debug=2    
interface=wlan0
bridge=br0
driver=nl80211
country_code=NL
ieee80211d=1    
logger_syslog=-1
logger_syslog_level=0
logger_stdout=-1
logger_stdout_level=0    
ht_capab=[HT40-][SHORT-GI-40][DSSS_CCK-40][MAX-AMSDU-3839]
auth_algs=1
hw_mode=g
chael=10
ieee80211n=1
wmm_enabled=1
ssid=redacted
wpa=2
wpa_psk_file=/etc/hostapd/wpa_psk
ignore_broadcast_ssid=0    
wpa_group_rekey=3600

I've tried various ht_capab stanzas -- including omitting it entirely -- to no avail. I've also tried configuring the network using a static IP address in CyanogenMod, but this too does not work. Using a WiFi analyser app reveals that this network is the only one on its chael, and provides the strongest signal of all nearby access points. The syslog on the router does not reveal anything interesting apart from periodic re-authentication.

Thanks in advance.