I have experienced simular issues with my Pi3 connecting to nodes, keeping time, updating Echolink online hosts all while using the onboard WiFi. What I found is that the DNS Name service IP was not being correctly applied to /etc/resolv.conf. See if you replicate the issues by following these steps:

Execute: cat /var/run/resolvconf/interfaces/wlan0

Result (yours will be different but should look simular):

domain hsd1.ca.comcast.net.

nameserver 192.168.240.1

Execute: cat /etc/resolv.conf

Result:

# This file is managed by systemd-resolved(8). Do not edit.

# Third party programs must not access this file directly, but

# only through the symlink at /etc/resolv.conf. To manage

# resolv.conf(5) in a different way, replace the symlink by a

# static file or a different symlink.

Execute: cat /etc/systemd/resolved.conf

Result:

# This file is part of systemd.

# systemd is free software; you can redistribute it and/or modify it

# under the terms of the GNU Lesser General Public License as published by

# the Free Software Foundation; either version 2.1 of the License, or

# (at your option) any later version.

# See resolved.conf(5) for details

[Resolve]

#DNS=

Execute: ping www.google.com

Result:

ping: unknown host www.google.com

If the above matches your case, I suggest doing the following:

Execute: resolvconf -u

(No result displayed)

Execute: ping www.google.com

Result:

PING www.google.com (74.125.135.103) 56(84) bytes of data.

64 bytes from pl-in-f103.1e100.net (74.125.135.103): icmp_seq=1 ttl=44 time=42.0 ms

--- www.google.com ping statistics ---

1 packets transmitted, 1 received, 0% packet loss, time 0ms

rtt min/avg/max/mdev = 42.074/42.074/42.074/0.000 ms

Execute: cat /etc/resolv.conf

Result:

# Generated by resolvconf

domain hsd1.ca.comcast.net.

nameserver 192.168.240.1

Now you can see that the nameserver is defined and DNS name resolution functions correctly.

Suggested perminent fix:

Add the following lines to /etc/rc.local just under the existing comments:

# Update DNS Resolution

$(which resolvconf) -u

Execute: shutdown -r now

After the reboot is complete, login and verify DNS resolves. A simple ping is enough to verify.

Please reply if this solves your issue.