Few potential culprits.
The cellular gateway might just not be getting all that great of a signal, especially if it's low down in a house and on a table or the floor. Might want to try moving it higher up.
The wlan end of the gateway might be a load of old shit, cheap 802.11 solutions often are, sometimes if it's old and cheap it'll get grumpy.
I know what a VPN is, it won't help you here, exactly what BRIDGE is I can only guess, but it's not going to save you either. You might also check the wlan card in the PC in question, if it doesn't have an antenna (full size card), the antenna has fallen off (mini PCI/PCIe card), or it's just not very good (either), those could also be culprits.
Also check your service plan, home internet via cellular is quite uncommon for a good reason, that shit's expensive, and they love to cut you off and throttle you, if you're hitting a data cap this might be why.