wifi drops after ~12 hours, cannot recover except by reboot

You can remove and reinsert the WiFi driver rather than rebooting:

/sbin/modprobe -v -r iwldvm # This removes iwlwifi too
/sbin/modprobe -v iwlwifi   # This starts iwldvm too
systemctl restart NetworkManager.service

The last line is optional and you can save time if it’s not needed. The above commands are for an Intel WiFi card. To get a list of your WiFi drivers use:

$ sudo lshw -C network
  *-network               
``` 
   description: Ethernet interface
   product: Killer E2400 Gigabit Ethernet Controller
   vendor: Qualcomm Atheros
   physical id: 0
   bus info: pci@0000:3b:00.0
   logical name: enp59s0
   version: 10
   serial: 28:f1:0e:2a:1a:ed
   size: 1Gbit/s
   capacity: 1Gbit/s
   width: 64 bits
   clock: 33MHz
   capabilities: pm pciexpress msi msix bus_master cap_list ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt-fd autonegotiation
   configuration: autonegotiation=on broadcast=yes driver=alx duplex=full ip=192.168.0.12 latency=0 link=yes multicast=yes port=twisted pair speed=1Gbit/s
   resources: irq:16 memory:dd600000-dd63ffff ioport:d000(size=128)
```
  *-network
``` 
   description: Wireless interface
   product: QCA6174 802.11ac Wireless Network Adapter
   vendor: Qualcomm Atheros
   physical id: 0
   bus info: pci@0000:3c:00.0
   logical name: wlp60s0
   version: 32
   serial: 9c:b6:d0:10:37:f7
   width: 64 bits
   clock: 33MHz
   capabilities: pm msi pciexpress bus_master cap_list ethernet physical wireless
   configuration: broadcast=yes driver=ath10k_pci driverversion=4.14.153-0414153-generic firmware=WLAN.RM.4.4.1-00079-QCARMSWPZ-1 ip=192.168.0.10 latency=0 link=yes multicast=yes wireless=IEEE 802.11
   resources: irq:137 memory:dd200000-dd3fffff
```

Notice the the WiFi information above in bold.

Then see if what is tied to the driver:

$ lsmod | grep ath10k_pci
ath10k_pci             53248  0
ath10k_core           417792  1 ath10k_pci

In this case all we need to do is remove ath10k_pci driver and reload it.

$ sudo modprobe -r -v ath10k_pci
rmmod ath10k_pci
rmmod ath10k_core
rmmod mac80211
rmmod ath
rmmod cfg80211

$ lsmod | grep ath10k_pci # Nothing appears so we've successfully removed driver!

$ sudo modprobe -v ath10k_pci
insmod /lib/modules/4.14.153-0414153-generic/kernel/net/wireless/cfg80211.ko 
insmod /lib/modules/4.14.153-0414153-generic/kernel/net/mac80211/mac80211.ko 
insmod /lib/modules/4.14.153-0414153-generic/kernel/drivers/net/wireless/ath/ath.ko 
insmod /lib/modules/4.14.153-0414153-generic/kernel/drivers/net/wireless/ath/ath10k/ath10k_core.ko 
insmod /lib/modules/4.14.153-0414153-generic/kernel/drivers/net/wireless/ath/ath10k/ath10k_pci.ko