Q:

Devices can't connect to server on LAN network

I'm better at programming than at network configuration, so this question might be newbi-ish.
My home wireless network is powered by a TP-Link Archer C20 v5 AC750 (192.168.0.1) and some devices are currently connected to it:

Dell notebook (192.168.0.104). This is "the server". It has an Apache web server (port 80), Plex media server, etc.
Android device (192.168.0.100)
Asus notebook (192.168.0.106)
Smart TV (192.168.0.105)

The issue is: mostly of the time, the devices are unable to connect to the server and vice-versa. But after the connection succeeds for the first time, subsequent requests are completed successfully.
Practical situations due to the problem:

When trying to use the Plex application on TV, it does not work (despite showing the server's IP in the "attempted connections" section of the error page). If I go to the server and issue a ping command to the television (ping 192.168.0.105) it start to work immediately and does not fail anymore until I restart the TV. Tried this several times and is a deterministic behaviour: the server always succeeds to ping the TV and Plex app works after this.
I tried to access a website in the Apache from the Asus notebook and it did not succeed. The server couldn't ping the Asus notebook either. Then I got the IPv6 address of the server and the client browser successfully accessed the server using the IPv6 address (http://[ipv6 address]:80/), but still cannot access (even ping fails) the server using the IPv4 address (and vice-versa: the server also fails to ping the client). I'm unable to ping the Asus notebook even using the "Diagnostic" tool in the router's management page. The Android device are unable to access the Apache even using the IPv6 address.
The Android device was unable to access the website in the Apache server using the browser, and I issued a ping 192.168.0.100 command from the server. It hung for a few minutes and then started to show packet responses in sequence (it clearly loss the first packets and then succeeded in establishing the connection). The Android immediately connected to the site after the first packet response appeared in the server. I stopped the ping command after this and the statistics were: 70 packets transmitted, 21 received, 70% packet loss, time 70228ms

What I've tried/checked so far:

DHCP distribute IP addresses from 192.168.0.100 to 192.168.0.199. Router (default gateway) is 192.168.0.1. Subnet mask is 255.255.0.0. Broadcast IP is configured as 192.168.255.255. DNS doesn't matter as I'm connecting directly by IP (but it is configured as well).
Router's firmware version is 0.9.1 4.16 v0001.0 Build 181213 Rel.33336n(4252)
All devices are connected to the same network (5Ghz)
The router's SPI firewall, DMZ, bandwidth control, and DoS protection are disabled
The IPv6 is disabled for the WAN (I didn't find any option to disable it for the LAN, I don't think there is such an option. The status page shows that IPv6 LAN Type is RADVD and the prefix size is 64)
Router's UPnP is enabled and Plex shows up there
The "Operational Mode" of the router is "Wireless Router"
The Apache web server is listening on 0.0.0.0 and is accessible outside of localhost
I pinned the IP of the server in the DHCP page, but the others didn't change either (all of them connect at least once a day and the address reservation time is 2880 minutes, the max allowed)

I don't have any clues on how the direct connection using local IP is failing and suddenly start to work (the third situation above, the Android device and server's ping to it), neither why it must be started in one direction (the first situation, on which the server's ping to TV solves Plex app connection issues).
Any advice would be much appreciated.

A:

The subnet mask needs to be changed to 255.255.255.0.

