Problems downloading from Google Drive over DrayTek routers

I'm having trouble downloading from Google Drive on two networks. One is FTTP with a Vigor 2762 and the other FTTC with a Vigor 2862. Both have the latest UK firmware. In both cases downloads (but not uploads) fail after a few MB and fail to retry using Chrome. (They also fail in Edge and Firefox.) If I manually retry the download it collects another few MB and stalls again. In the case of the FTTP network these downloads work fine if the router is swapped out for the one supplied by the ISP. Is this a configuration issue on the DrayTek? It seems to be the only thing that doesn't work as expected!

I can't comment on other routers fixing it but I've known many problems with Google Drive over the years.

A different browser (not signed in to Google) has sometimes fixed it, but only if the share doesn't force a Google sign in.

My suspicion (and it's just that) is that it's in some way related to how much space you have free in your own Google account.

Thank you, @markhawkin, but the issue appears to be nothing to do with Google Drive. I've changed to a TL-ER7206 and everything works fine. The UI reboots really slowly but requires far fewer reboots on config changes.
After several time-consuming (and mostly unresolved) problems with DrayTek routers, including the more expensive v2862, I'll have to stick with this.

Now if more people were saying they were having issues with google drive and Draytek products I would agree with you and say it's a Draytek issue. Since this isn't happening it's going to be something odd with your setup with the TP-Link isn't bothered about. 

I've had dual wan until recently and various draytek routers and never had a google drive issue. Issues with other stuff buggy firmware. Any other issues were on the windows side.

I would love to agree with you and would be sad to lose my investment of both time and money in DrayTek products but it's hard to know what to do. The different behaviour is shown by simply plugging a direct ethernet cable between my laptop and the different routers: TP-Link (bought) and Plume (supplied by the ISP) work with G Drive whereas DrayTek doesn't.
In each case the router supplies DHCP. There are no weird tweaks to the Windows networking. However, I did try winding down the MTU, etc. on the DrayTek but without success. The problem is shown with both 2762 and 2862 using the latest firmware. It also affects other Windows devices on the LAN.
Do you have any debug suggestions?

@markhawkin : Your latest post about IPv6 (or lack of) possibly causing this issue has disappeared! Along with my reply to it. Would you repost, please?