I have the same problem / behaviour (losing connection to 443 for 1-4 seconds) over here in Italy
I have already changed and tried a lot:
2 different providers (vodafond, ho-mobile)
2 different routers (d-link mobile router, cudy with "good antennas") + 2 different hotspot-smartphones
other connected devices (without pulsesecure vpn) do not show any problems
IP-Address (from Provider) does not change (maybe every day but for sure not every 2 minutes)
Contacted "Three UK" client support and I have been told that the IP should not change at least or a few hours, they said maybe 1 or 2 times per day at max. But my disconnection is every 3 minutes. VPN drops and reconnects after a few seconds.
I suspect the issue is related with this:
Multicast routing devices running PIM sparse mode often forward the same stream of multicast packets onto the same LAN through the rendezvous-point tree (RPT) and shortest-path tree (SPT). PIM assert messages help routing devices determine which routing device forwards the traffic and prunes the RPT for this group. By default, routing devices enter an assert cycle every 180 seconds. You can configure this assert timeout to be between 5 and 210 seconds.