Hi,
Glad you have good results with EE, with an external antenna you'd probably get significantly better results.
The Teams problem happen with this sim, too?
Can you check the MTU settings in both routers, are they the same? ( Not sure where you'd find this in the menu, but it's there, at least for Huawei).
There are a few things to try, though not without some risks:
1 - go in the adv settings and disable everything in the Firewall section
2 - put router in bridge mode from Router menu, this would enable you to use the archer as wifi & dhcp router: remove sim from archer, plug its wan port in the b818 lan port ( also read the instructions, as this will lock you out of the huawei i think)
If you have some older wifi router lying around, you could use that as well instead of the archer.
3 - put your wife's pc in the dmz host or virtual server in the b818 settings, that's almost as if having a public ip address, connected to the internet without a firewall, so make sure the windows firewall is on. The risk may actually be lower as EE uses cgnat in their infrastructure.