I am currently working with a BT HH5A running the latest firmware as of this date. Before I deep dive into this, may I just ask whether the WireGuard implementation on OpenWrt is a bit temperamental?
I had WireGuard installed and working fine (after a bit of fiddling with the zone and forwarding rules) with my Android phone as a client both on 4G and external wifi networks, such as on public transport, then I added a profile for my laptop and neither device now works.
During my earlier fiddling with WireGuard I got to a point where I could not connect to the HH5A and had to reset it and start over.
So, in summary, do I need to start poking around with the config files to see what's occurred or am I just unlucky. Setting this up, while following the numerous guides on the subject. seems to have been more full of pitfalls than I expected - or is it me!?
I have had Wireguard running on BT Hub5 for couple of years. All the setup problems came from the firewall rules. I have multiple devices connecting without problem once I got the first device going. Connecting my android phone was the easiest. Good luck with the firewall.
Probably your configuration has something wrong, or your network like firewall/routes. WireGuard is great, I use it on dozen of connections and works always perfectly. Just sometimes from the menubar on macOS doesn't work and you have to open the full interface and click on "connect", but is not a network issue.
At work, for example, I have a Mikrotik router (with OpenWrt that acts only like as WireGuard server) behind a FortiGate firewall and I had to configure also a static route for WireGuard, where 10.4 is the WireGuard interface and 192.168.113.4 is the Mikrotik/openwrt router: