To be honest - no, I doubt that will work.
You can achieve a failover-config, but a link aggregation cannot speed up a single connection you establish to a content provider.
To get higher download speeds from one single source this source would need to offer multiple connections at the same time. Which is - apart from speed tests and specific connection protocols (torrent, ones clicks boaters, etc) - quite rare.
The professional environments I got to know in the past had all multiple fibre links from different ISPs - for failover reasons.
Besides this theory there is (to my knowledge) only mwan3 package for failover available, but no package for wan link aggregation…
So instead of (dreaming of) aggregating VDSL+ LTE, better get a 5G-contract with unlimited data volume or even better directly a starlink subscription.
if you think only a fritzbox will help you out, you are most probably mistaken.
it´s either your modem OR the settings you use.
both will be “fixed” with a FB, because it has it´s own modem and a different UI.
why don´t you just try a different modem? should still be cheaper than a fb. (and if it doesn´t work, just return or sell the modem…)
and why do i think it´s the modem? because i use a modem (zyxel vmg4005) + omnia on a dsl line in germany from a telekom reseller (ewe tel partially uses telekom lines) without problems. works like a charm for almost 8 months now. and all i did was seting the “modem” to bridge mode and entering my credentials in reforis. that was it.
why should a “router behind a router” be significantly / noticeably (!) slower? the small overhead in some cases aside, i do not really think you´ll notice that.
Well yes and no. There is this multipath-tcp project
that is actually trying to make use of such a scenario. And mwan3 package can be used to balance between two links instead of use fail-over which could also gain a bit depending on the link quality.